MA790FXT-UD5P and failing ethernet?

I bought this MA790FXT-UD5P board several months ago and have been using it as a workstation, and other than a few bios issues, everything has been great. Unfortunately as of last week the ethernet ports have started to act up. First the port I was using started to take an age to get an IP (DHCP request), then it would fail several times in a row, during all that it failed to work at GbE, and eventually won't even work at 10mb/s. I've even tested with the BIOS port tester utility. As of right now, I'm stuck with 100BaseTX (full duplex), which feels more like 10mbps. I don't seem to have any lost packets, at least none that the OS can see, but it is horribly slow.

I've tried several different known working cables. They all work fine at GbE on my laptop, but any attempt to change the mode to GbE results in a dead interface.

Have I missed something? Or is my motherboard dying?

I've since updated to the latest bios (F7) and it still hasn't helped any. DHCPDISCOVERs still only work if I try over and over, while resetting the nic (/etc/init/networking restart).

On A slightly related topic, my MA790FXT-UD5P will only boot from total power-off (i.e. no mains power to the PSU) if I first remove the network cable, and then replace it after the first boot. After that the m/b is happy to boot with it connected. This is with BIOS F7. I too have had a few problems with networking failing, although my network is still 100Base-TX.

Check your PCIe frequency in the BIOS; you want it set to 100, not 'auto' - the divider mechanism that sets it is buggy, and if you're OC'd anywhere, it's likely to be wrong - and the first thing that 'drops off' the bus is usually the RealTek RTL8111DLs that run the LAN stack...