I built my own system (2.4Ghz quad overclocked to 2.8, 8600 nvidia dual-link, asus P5K deluxe wifi) and normally it works like a dream!
But sometimes I boot it and the system and hdd lights stay lit.... but no POST beep occurs and no graphics show up. This morning that happened and I turned it off and flipped the power in the back off and back on... and now it keeps rebooting at the point it SHOULD be turning on my monitors once they get a signal.
This happened before when I was building it and a motherboard reset did it. But what can I do to help prevent this from continuing??!
I'd clock everything back to factory speeds and see if the problem goes away. If that solves your problems, you'll need to verify that your RAM is capable of the increased speeds and that your PSU can handle the extra load.
I've tried all of the above. Resetting my motherboard, reseating my video card and ram. The strange thing about this is that my computer worked just fine last night. It's just sometimes (and usually in the morning :|) that it doesn't boot the first time. this morning it did it twice in a row. And after waiting for it, I decided to cut the power in the back and try again. So I did that and now it's just restarting after it starts.
I had sorta the same problem with my Asus A8N-VM CSM motherboard. To fix it, I had to update the bios, then turn off Cool 'n Quiet as well as QFan. Seems the CPU fan was spinning too slow to register because the CPU wasnt warm/hot and the system thought it was a fan failure and would reboot or shutdown (or sometimes not even post) on me on a cold boot.