Computer losing power, failing to power itself back on until unplugged/replugged.

I wasn't exactly sure which subforum this post would belong in, and I know this isn't exactly a troubleshooting forum but I need help.

Two weeks ago my PC randomly started suffering from shutdowns where it was immediately lose power, and then I restarted it... it would cycle on and off every few seconds until I physically removed the power plug from the motherboard and reinserted it.

First I thought it was the PSU, which is a 1000w Mach, and I replaced it with a smaller 600w and still the issue persisted. There was no smoke or any other visible cue which would lead me to suspect that it was a PSU problem.

Then I thought it was the CPU. I have a Q6600 with a Zalman heatsink. I downloaded speed fan and the temperatures were fine, I never saw them go above 55-65c.

Then I thought it was the motherboard. I bought a brand new EP45 and rebuilt my computer, installed my CPU and installed my heatsink, cleaned off all the dust, new case. etc.

When I went to install the operating system, it started happening again...

Then I tried doing it with different sticks of ram, and still... it persists. The only thing I haven't checked is the video card, but that wouldn't cause a system malfunction or a power cycling problem.

Something is overheating and I don't know what it is.

Bad CPU maybe? Try it in a different room, you might be pulling too much on a dieing breaker/wiring in your walls. Make sure the other room is on a different circuit as well.

would it be normal for only the pc to shut off?

Well it was the video card which is an EVGA GTX 260. My PSU's have internal safety mechanisms where they shut off in the event of either over-heating and over-voltaging.

Both 4x2 molex pins were connected to the card, and it had run fine for 2-3 months, and the fan is still spinning. Anyone know why this is happening all of a sudden? It doesn't appear that the card is defective.

How did you figure it was the video card?

I just had a very similar problem with my nVidia 8800 GT after upgrading to a new system (I was going to use the 8800 GT in the new system). In the new system it caused Windows 7 to freeze randomly. I finally figured out it was the video card and not something else (after a week of tearing my hair off and trying everything else I could think of) by putting it in my old system: it caused the old PC to continuously turn on and off at boot.

I also wonder exactly what the card was doing to cause the freezes in the new system and the continuous reboots in the old system. The weirdest thing is that I could even run 3D Mark 06 with the 'broken' 8800 GT on the Windows 7 PC - it would just freeze or reboot the PC at random times, other than that it seemed to be operating normally.