I am experiencing a simliar problem with my PC. It has been getting progressively worse as time goes on and normally occurs once the PC is warmed up. Reboots are not recommended on my machine at the moment. I have located my problem to the PSU. The POWER GOOD signal is not coming up on the 24 pin power connector. I have ordered a new supply.
Has a similar problem recently when doing a full reboot and thought it was the power supply but turned out to be a dead memory chip that was stopping the machine getting past POST. It was Rendition memory - not the first time I have had problems with their memory failing