H55M-USB3 dying?

[Senseotech]

Several months back, I did the Build-your-own Airport card, and had zero luck; the card worked for 7-10 minutes, then lost all networks until a reboot. It did this in Win7 and SL/Lion, so I assumed it was either the card or adapter. Bought another of each, same results. Fast forward to about a week ago, and I leave the room, only to come back about an hour later to a powered down system that wouldn't power on until I unplugged it and plugged it back in. Once I got it back up, the BIOS informed me it had been reset and that I needed to pick a previous configuration. This happened again the next day, and the next. It didn't matter wether it was OS X or Windows, randomly during the day, I'd leave and come back to a machine that was down. Just on a hunch, I tried a power supply tester, but the PSU is fine. Between these sudden {censored}-outs, and the problems I experienced trying to add an airport card, I'm beginning to think its the mobo going down. Any suggestions before I go out and drop $300+ on a Sandy Bridge setup?