I've also had problems with the onboard ethernet card failing intermittently. I'm guessing all this started when I put a more powerful graphics card in (ATI Radeon HD 4670). To be honest, I actually haven't noticed any problems with the GPU, but then again, I haven't really used it that intensively.
The PSU is only 250W (peak 300W) so I'm guessing that's why components are turning off(?) intermittently. Is there any way to prove that the low PSU output is the problem, other than buying a new one.
I'm going to order a new one anyway since this one has more 4-pin molex connectors than 15-pin SATA connectors (which is very odd). Also, they aren't that expensive, but I figured I'd put this out there just in case.