Recently came back from an extended trip in Michigan to my home in Florida, only to discover that my Mac Pro is randomly restarting both with my Windows Partition and Mac Hard Drive. Restarts seem to be occur quicker when playing a Windows Games, or doing some heavy loads while on the Mac side - though it can and will happen if I play some World of Warcraft on the mac. The computer restarts, no kernel panic, just simply restarts. On the Mac side of things it will relaunch the application I was running (even though I have setup Lion to not do this). System has a brand new AMD HD5770 installed, I have cleared out the dust in the system, though not going so far as changing the thermal paste on the CPUs/Heatsink - overall the system should be fine. I suspect that the issue is the PSU, but it being the Holidays I'm trying to save money instead of wasting it to take this system to the local third party Apple approved repair shop. The question I have is this - am I correct to think this is the PSU needing to be replaced? Is sudden restarts a symptom? Or is this an overheating issue? I have run Apple Diagnostic Disk multiple times, not experiencing any of the random restarts that occur when actually using the system.