This is (see attachment) what the contents of that ZIP file look like. If you tell me which of these fils to attach here I will, I just didn't know where to start and that is what the Wireless Diagnostics app creates.

Notice the dates on the files.
The only files that are relevant are the ones from the time when you did the test. I.e. likely the most recent files.

For a start, system.log

Trevor

Ok I have uploaded the system.log, the wireless diagnostics log and the wifi log limited to the last 2 days
i hope these will provide a cue

Nothing jumps out at me in those logs as indicating a problem.
I think to track this down, you need to take careful note of the time when the problem occurs and tell us the time and supply logs from that time.

But it would also be useful if you were more precise about what the problem is - what exactly are the symptoms?

I will try to jot down the times but there are no symptoms, connection just stops working, nobody home... if I have the Airport Utility window open I see the internet world icon go from green to yellow

At 10.11 the internet went out and I launched Wireless Diagnostics
Between 10.11am and 10.17am the internet was basically out and consistently (if you loaded a page it would either not load at all or start loading and stop loading right away or display a broken page, maybe from cache? i don't know)...
Because Wireless Diagnostics restarts itself every time a connection is dropped I had to restart it several times so I am uploading multiple reports of that time period.
In the files you'll see multiple System logs, Wifi logs and Wireless Diagnostics logs with names like System.log system copy 1.log etc... they are not copies, they are the files created by multiple runs of the Wireless Diagnostics app.
Thanks guys!

Those logs (esp the wireless logs) show that you seem to have lost "WAN connectivity". I.e. that your Mac could not reach the wider Internet.

Now you need to find out if the problem is with the modem. Did the modem lose connectivity with the Internet at that time?
To find this out, you need to get access to the logs of the modem or else have been looking at the status lights of the modem at the time of the problem.

Or we can also use that longer cable and use a wired connection for a sustained period.. Did we do that already?

I've been using the Cat5 cable for the last two days and haven't had any noticeable drops of connectivity. I connected the ethernet cable to the back of the Airport Extreme.
I guess that means it's the Wifi signal itself, but what does that mean exactly and how can I fix it?

So to clarify, you've had your Mac connected via Ethernet to the Airport Extreme and had no problems - is that correct?
That would seem to indicate a problem with the wireless signal from the Airport Extreme.
But just to be sure, you previously were connecting via a wireless network provided by the Airport Extreme? And your modem is not providing a wireless network which you might have accidentally connected to.
Btw, you do have a password (preferably WPA2) on your wireless network?

I believe Hayne your summarizing correctly, OP of course can say best. Agreed we need to know also if the modem is also throwing out wireless that might interfere.

If I am recollecting correctly the distance is close so the main two possibilities are interference or buggy wifi software, not sure if there are generally current issues. I think all have problems at least periodically. ** Interference can be caused by other wireless device, microwaves, garage door openers, alarm systems, etc. Since the distance is line of site/fairly short without walls. Lets make sure your on the 5gz band which is much more immune to interference then 2.4ghz.

All correct.
Wire seems fine. Wireless drops out.
No garage, microwave or alarm systems anywhere near.
Modem does not do wireless and I do have a wpa2 password on the signal generated by Airport Express.
How would I switch to 5GHz?

One other consideration (not sure if that helps). I seem to have noticed that it seems to drop out more when both myself and my partner (on a PC) connect and work on our computers. Does that mean anything?

Thanks guys

Do not assume not close interference things means they do not cause issues. Do not forget also about other wireless routers, wireless phones, baby monitors, etc, etc.

Quick and Dirty you can hold down Option key and then go to Wifi Icon near the the top right of Menu Bar to the Right of blue tooth Indicator. This will give you a lot of details on how your connected at any one moment.

You Extreme does simultaneous Dual Band 2.4 and 5hz both... If your not automatically connecting to 5ghz you can force it. Go to Airport Utilty, Go to Wireless Tab, Wireless Options button, Check the Radio Button for 5ghz Wireless name and give the 5ghz band a different wireless name so that you can specifically connect to it.

Generally, this manual 5 ghz selection is not necessary and can cause minor hiccups since you are forcing a connection to 5ghz band which if it is to weak will cause a disconnect or poor browsing until you manually switch to 2.4ghz, when you do not specify separate names your suppose to hop back and forth automatically as needed.

In your situation it should work well manually selecting 5ghz or at least be an interesting test. Note 5ghz is faster and less prone to interference. However, 5ghz is much less able to penetrate walls or travel longer distances.

As to what you noticed about another client. Generally more wireless clients should not be an issue and your router is simultaneous dual band. However, if perhaps your on 2.4ghz more when your partner is online that might explain the bad behavior since that band is more prone to interference.

It could mean that your configurations (on one or both of those computers) is incorrect. For example, both of the computers could be trying to use the same IP address - that would cause huge problems. Check that the computers are set to use DHCP and that the DHCP server is the correct one - presumably the Airport Extreme.

I just tried switching the network in Airport Utility to 5Ghz only. I am on my Mac Book Pro and my partner is on her iPad and after about 10 min of internet on the 5Ghz there was a dropout again for a few minutes... I'm really at a loss

I am assuming you mean you set up in airport a 5ghz only option and name and you choose that option from your macbook Pro, and you still have disconnects.

I didn't see the option to create a separate secondary network so I selected 5Ghz only from the drop down menu in the wireless options and the AirPort Extreme went offline for a moment to update it's settings and then came back broadcasting at 5Ghz (I checked by option-clicking on the wifi icon as you had suggested)

Did you?
 

"Your Extreme does simultaneous Dual Band 2.4 and 5hz both... If your not automatically connecting to 5ghz you can force it. Go to Airport Utilty, Go to Wireless Tab, Wireless Options button, Check the Radio Button for 5ghz Wireless name and give the 5ghz band a different wireless name so that you can specifically connect to it."

Verify 2.4 vs. 5ghz connection

"....Hold down Option key and then go to Wifi Icon near the the top right of Menu Bar to the Right of blue tooth Indicator. This will give you a lot of details on how your connected at any one moment."

Hayne's thought about about DHCP and duplicate IP addresses is a possibility too but you need to test one theory at a time in Science..

I had read the instructions the first time but I find now Radio Button to create a 5Ghz wireless name in my Wireless options (see attachment to see what I see) so I just set the Radio Mode to 5Ghz only...