I don't have much experience with them myself. All I know is that you're having issues with all of your PCI slots, so the probability is low that they are the root problem. You've tried multiple cards, so again, the probability is low that they are the problem. You've done all the standard troubleshooting. (I assume there are no uninstalled firmware updates.) Checking to see if the power supply is producing and the PCI slots are receiving sufficient power is the only other thing I can think of. I don't know if there is a difference between the PS in your Mac and a comparable PC's PS.

There are differences in the PSU's between Macs and PC's.Shape is the biggest. You need the external dimensions to be the same size or smaller and mod it to fit with brackets etc if using a non standard one. The pinouts also are different. ATX have been modified to work in some Macs but from what I remember it involved some soldering.
Stewie

As to the USB card... I've never had any luck with USB 2.0 cards in Macs unless they used the NEC chipset. The NEC should work without third party drivers.