Computer Shutdown then Unresponsive

My computer has been suffering from this problem for awhile. When in use sometimes it will reboot itself with no error message and continue to reboot until I have to shut it down manually. When I try to restart it the fans and external lights will come on (as well as temp guages) but the hard drive and other drives will not come on, no signal will be sent to the monitor either. I usually will not be able to restart it for a day or so, then I can get it working fine again for a couple of days until the problem repeats itself.

Efforts I have been through are detailed, to include changing RAM, switching out all PCI, and replacing video card…nothing has worked so far. Can someone help me track this problem down?