Ok, I set the PC up as an FTP server on the LAN and was able to upload and download files from the Mac using an FTP client.

So, does this then tell me with 100% certainty it's the internet provider causing these problems? How is it possible a PC can connect to an external FTP site without any problems but a Mac can't?

Pretty much, yes.

There may be some slight difference between the packets the Mac is sending and the PC packets that makes the Mac's packets more susceptible to error. If the Mac packets are larger for example, you could expect a slightly faster transmission on a solid connection, but more packet failures on a marginal connection.

1) You do realize that your ftp passwords (and maybe other passwords) are embedded in that transcript of packet captures that you are showing us. I hope you changed your passwords after showing that to us (and to the world)!

2) It seems that you have AppleTalk enabled on your network - why? AppleTalk isn't usually needed except for quite old printers etc.

3) How is your Mac connected to the network? Is it connected via Ethernet or via Airport?

4) What is the MTU value that you get from the following command?

ifconfig | grep mtu

(Show us the MTU for whichever network interface is relevant for this Mac (en0 = Ethernet, en1 = Airport)

1) I didn't realize, and I did change them.

2) Since having an old Centrus 610 I've always left Appletalk active by habit... not really even considering it's unnecessary.

3) Via airport

4) en1: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500

Thanks for your help (and the heads up)

Is the PC connected wirelessly too? If not, there is the possibility of wireless interference affecting your signal being the culprit.

Yeah, PC's wireless as well. To rule out the wireless option I tried hard-wiring both Macs and the PC to the modem directly and still the problems persist with the 2 Macs.

The reason I asked about the wireless connection is that it is often necessary to set the MTU lower than the default 1500 when using wireless - this depends on your ISP. Search the forums and the main macosxhints site and you will find lots of discussion about how to set the MTU for Airport.
I have mine set to an unusually low value of 1364
(via the command 'sudo ifconfig en1 mtu 1364')

Tried various settings in the MTU field and no luck.

Any other ideas? Any clue based on the report I posted here from the Ethereal report?

FYI, I tried connecting via a friend's wireless connection (different ISP) and it worked perfectly. cwtnospam called it on this one, yet I still don't understand exactly how an ISP can allow FTP communication on PCs but not Macs... ANy tweaks possible on a Mac-based FTP program to allow me to send via FTP on this connection?

Is there an option to use (or not) a "passive" FTP connection? Try toggling it to whatever setting isn't currently in use.

You might try setting the MTU on en1 to something like 1200 or 1300.

I wasn't saying the ISP is blocking you, just that your connection to them is getting enough errors that the Mac's packets aren't making it through often enough for the Mac.

I agree that you should try again with changing the MTU setting for Airport.
Please show us the result of 'ifconfig' after having changed the MTU.

Hopefully by posting this I'm not sharing my SSN or other vital data, but here it is anyway... the "ifconfig".

- - - - - -

lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 16384
inet 127.0.0.1 netmask 0xff000000
inet6 ::1 prefixlen 128
inet6 fe80::1%lo0 prefixlen 64 scopeid 0x1
gif0: flags=8010<POINTOPOINT,MULTICAST> mtu 1280
stf0: flags=0<> mtu 1280
en0: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
ether 00:17:f2:d1:86:7c
media: autoselect status: inactive
supported media: autoselect 10baseT/UTP <half-duplex> 10baseT/UTP <full-duplex> 10baseT/UTP <full-duplex,hw-loopback> 10baseT/UTP <full-duplex,flow-control> 100baseTX <half-duplex> 100baseTX <full-duplex> 100baseTX <full-duplex,hw-loopback> 100baseTX <full-duplex,flow-control> 1000baseT <full-duplex> 1000baseT <full-duplex,hw-loopback> 1000baseT <full-duplex,flow-control> none
en1: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
inet6 fe80::219:e3ff:fed3:700f%en1 prefixlen 64 scopeid 0x5
inet 192.168.0.101 netmask 0xffffff00 broadcast 192.168.0.255
ether 00:19:e3:d3:70:0f
media: autoselect status: active
supported media: autoselect
wlt1: flags=41<UP,RUNNING> mtu 1500
fw0: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 2030
lladdr 00:19:e3:ff:fe:6e:00:f4
media: autoselect <full-duplex> status: inactive
supported media: autoselect <full-duplex>

I think Hayne intended for you to use:
sudo ifconfig en1 mtu 1300

from a Terminal window in an admin account.

This line:
en1: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
indicates that if you used it, it didn't take.

I had changed the MTU in the router. Here's the result after:
sudo ifconfig en1 mtu 1300

- - -
en1: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1300
inet6 fe80::219:e3ff:fed3:700f%en1 prefixlen 64 scopeid 0x5
inet 192.168.0.101 netmask 0xffffff00 broadcast 192.168.0.255
ether 00:19:e3:d3:70:0f
media: autoselect status: active
supported media: autoselect