Hmmm. Starting to sound a little like a dry connection on the board or similar. If it is, it'll be effected by temperature & time. Try leaving the PC powered off for enough time to cool right down. Hopefully it'll boot fine. Over time and with the temperature rising you may see the faults coming back and fans spinning up & down. Immediate reboots may / may not work and if you've checked the ovious things like Video cable, PSU, POST etc then my best guess is get to a good PC shop to check the solder / components.
