Over the last few days I've had a couple random shutdowns with my early 08 Mac Pro. Once was during gaming in OSX, the other just surfing in Win7. Both times the system has just simply shutdown, no errors at all, but then the video card fans stay on at max speed. On the OSX side of things I have recently upgraded to Lion and installed the 10.7.1 update. Nothing else that I can think of. On the Windows side I've installed a few updates through Windows Update, but that's about it. Off the top of my head I'm thinking it's either the power supply or the video card maybe? What's the best way to go about testing either of these as possible culprits?