Computer powers off and restarts on hibernate/sleep/restart
Occasionally, my PC (Windows 7) will shut off, then after a second or two, power back on. This happens when I try to go into hibernate or sleep mode from Windows 7, or sometimes it will do this when I try to restart it (that is, instead of doing a clean reboot where Windows shuts down, then I see the BIOS screen, then Windows boots up, it will actually power off and then restart). When Windows is actually running there are no problems.
If it does this on a hibernate or sleep, when Windows boots I get the 'Windows did not shut down properly' screen, and if I select 'Start Windows normally', it boots up and everything seems OK.
There is never a blue screen, and there are no events listed in Windows event log (apart from a critical error that says Windows stopped without shutting down properly, which I already know). It looks like the power is just cut so that Windows doesn't have a chance to register a problem. The problem is intermittent, as sometimes the machine can go for a fair number of hibernates without any problems, then the problem will happen again.
A friend suggested this might be due to a faulty power supply; does that seem likely?
Thanks for any advice.