Great that you have solved your problem. Mine, I am afraid to say that it has gone from bad to worse. I reported earlier that it will be stable after the first sudden shutdown but now, it is not so, it can shut down again.
I noticed that I am not able to boot up again after the shutdowns, unless, I hold the power button down for at least 4 seconds (just like forced shutdown a PC), release and then re-press the button. I got a feeling that the hardware (that manages the power up/shutdown) still thinks it hasn't shutdown, so pressing the power button does not do anything. There are times, it will just flicker (LEDs) when I try to re-start and then dies away immediately.
When that happens, sometimes, it will never boot but I found and narrow down the culprit. If I nudge the RAM sticks, it will boot up. I am beginning to think that it is the RAM slots or the circuitry around it. I have tested the RAM with MemTest and they are all ok. All 4 slots are filled to get 2GB, and they are paired up RAMS.
The occurrences happen more frequently when the weather is cooler and I think this suggests some connection issues between the RAM and the slots. Once everything heats up, it will be stable. And this discount many heat related failures (to the PSU).
I seriously think that it is the mobo and at some percentage, the PSU. I am going to get a good working PSU to try if it is the PSU. First thing to do is up the RAM voltage to 2V. I think I've done this before and still experience the crash, but let me try again, perhaps my mind is playing tricks on me.
My old rig is as follows:
Gigabyte GA-P965-DS3
512MB X4 --> 2GB Dual Channel (set to +1V)
160GB SATA drive
SB Audigy 4 SoundCard
8800GTX video card (not oC)
E6600 2.4GHz o.c to 3.06GHz at 1.41Vcore (used to be at 3.3GHz)
Using CoolerMaster heatpipe Gemini S (5 heat pipes)
Multiple fans (about 9 fans running)
PSU = Cooler Master OEM 430Watts (380Watts continuous)
My last resort, upon confirmation is the Mobo is to get a Asus G31 mobo, something inexpensive as my ultimate goal is to get the i5 750 system later on. I plan to give away this current rig as everything is still adequately fast.