Configuration:
Processor- Intel Dual Core E5700
GPU- Internal
Cooler- Stock Intel LGA775
RAM- 2GB Kingston 1333MHz / 2GB EVM Dynet 1333MHz
Mobo- Gigabyte GA G41M Combo
HDD- Seagate Barracuda 500GB
PSU- 400 W Circle Power Supply
Case: Some generic no brand case
OS- Windows 8.1 64 Bit
 
Have run with this config (except the new PSU) for a couple years without issues. For the past one month my PC has been randomly turning off without warning or BSOD. Sometimes while booting and after running for hours at other times.
 
What I have tried so far:
1. Replaced the old PSU with leaked capacitors with a new one.
2. Replaced dead CMOS battery.
3. Cleaned the heatsink and re-installed it with new thermal paste. (Temps normal now while idle ~40 deg c and under load ~72 deg c)
4. Reseated the Processor.
5. Tried one RAM stick at a time and both.
6. Checked the PSU voltages. Fan runs normally with GREEN and Black pins shorted. Here are the readings: All Orange Pins: 3.39v, All Red Pins: 5.37v, All Yellow Pins: 12.19v, Gray (Pin : 5.35v, Purple (Pin 9): 5.25v, Blue (Pin 14): -11.62. The Red pin voltages have me a little worried because according to this article the max voltage for Red pins is 5.25v.
7. Tried breadboarding. PC ran fine for few hours but it runs fine for a few hours in the case as well. So I think breadboarding was inconclusive.
 
The BIOS speaker beeps once while booting which indicates all is fine with my Award BIOS. One of my mobo standoffs is missing but I put a cardboard between the mobo and case, so I don’t think the mobo is shorting on the back. Also cleaned the mobo as much as I could. I have no overclocks or under volting. There seem to be no loose screws lying around. All cables are plugged in properly. Another thing worth noting is that once it turns off, pressing the power button on the case won’t start the PC. I have to UNPLUG the PC from the wall socket and plug it back in before it can turn on.
 
Someone help me out here. If more info is needed I'll add it.What make and model PSU did you replace with ? ?

Meantime i'd get a few standoffs from a shop....they usually give them away.Cooler- Stock Intel LGA775

I've seen these shear at the plastic standoffs (where the spring is).  Not visible until you remove it & visually inspect close up.  It BREAKS the contact between the cpu, thermal paste & the cooler just enough so it's not in full contact which causes very fast overheating & shutdown.

Sometimes you can tell because the fan speeds up at strange times.  Is you fan speeding up before before the shutdown? Quote from: patio on September 30, 2017, 11:38:12 AM

What make and model PSU did you replace with ? ?

Meantime i'd get a few standoffs from a shop....they usually give them away.

Cooler- Stock Intel LGA775

I've seen these shear at the plastic standoffs (where the spring is).  Not visible until you remove it & visually inspect close up.  It breaks the contact between the cpu, thermal paste & the cooler just enough so it's not in full contact which causes very fast overheating & shutdown.

Sometimes you can tell because the fan speeds up at strange times.  Is you fan speeding up before before the shutdown?

I checked the heatsink locks and they are properly locked in. The heatsink doesn't move either. There were never any temp spikes and the CPU was stable at 72 Deg C with 100% load. And I have never noticed the fan speed up suddenly.