Computer Shuts off randomly at cold boot (after hours unplugged) but starts perfectly after the "shut off"

Computer shuts off randomly (after hours unplugged)
Scenario:

1. Computer shuts off on cold boot (after hours unplugged) at Windows desktop around 2 minutes, also at bootable memtest.
2. After the shut off - powering on the PC again and it runs perfectly (passed all burn-in test). Runs pc games perfectly 8 hours to 10 hours. Ex. Wasteland 2

Troubleshooting I made:
1. Cleaned PC thoroughly using compressed air.
2. New thermal paste applied on the processor.
3. Video card and Memory contact cleaned with rubber eraser.
4. Reconnected all cables and checled fpr loose cables.
5. Bought a new power supply. (Deepcool DE530 400wattnrated)
6. Removed all hardware. Only the Processor and motherboard are running.
7. Memtest memory at boot up. Passed. No errors.
8. New CMOS battery
9. Bios jumper reset at motherboard.
10. New Windows 7 home installation. Dual boot removed. (Linux)
11. Capacitors not bloated.

After troubleshooting it still shuts off randomly at the first cold boot.

My computer specs. (This is an old PC, at 7 years old) :-D

Processor: AMD Athlon X3 435
Motherboard: Asrock N68-S-UCC motherboard.
DIMM: 2x 2gb DDR3 memory.
ROM: Asus DVD rom RW
Nvdia GT630 DDR3
Old PSU: Great Wall Black series 450wl (old)
New PSU: Deepcool DE530 rated 400 watts

The Motherboard is already failing/faulty?
Will just need to buy a new motherboard?
Did I miss something?

(I live in Asia, some shops still have brand new motherboards for AM3 processors.)


I know this is an old PC, but I still want to keep it around. PC had a some kind of a sentimental value.

Thank you very much.
Reply to darth.gabby
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