I don't have the foggiest idea what is causing this. The computer in question just basically shuts off, but all fans are still running, I pressed the power button and it boots up. Seems like it could be a thermal issue but that doesn't make a lot of sense, I haven't mucked about inside it for a while. Power supply is next logical answer, but it was replaced a year ago with a nice Seasonic unit. Ran memtest with no errors. any ideas?