I know it sounds REALLY weird, but how about turning off your router when doing heavy tasks? I know that sometimes, at night when my PC was Hibernated (not shut down - using hybrid sleep) the thing would wake up at night because, for some reason, the Gateway router from Comcast wanted to use my computer at night. So, try doing a heavy task with the router off. If it restarts (that's what mine would do), then you can cancel out another possibility.
BTW, I managed to change the setting that allows the router to do that in Power Options. I know this is an issue as I always seem to forget about it when I've installed/reinstalled Windows.