Intermittent PSU fault (maybe?) help please

[AWPC]

Please can anyone help me here. I have tried the following:

Memtest run all tests for over 2 hours no errors whatsever.
Played GPU intensive games for 5-6 hours with perfect stability (Nvidia GTX 780 Ti @ 2560x1080 max details).
Intel Processor Diagnostic Stress tests.
System cooling is always ok when this happens no temp spike the CPU is around 35-40C away from the max TDP.
No viruses anywhere.

Now sometimes its impossible to tell when but I suddenly get a reboot always without warning whilst on the Win 7 x64 desktop or using IE 11 to browse.

Also now & again I have to from cold restart the PC as it does not initialise the Intel Lan port first time from cold but 2nd time it works fine.

I am thinking either the following:

Motherboard is failing & or has faulty caps.
CPU is failing.
PSU is failing (Seasonic 750W X series 80 plus Gold just over 2 years old).

The motherboard & CPU are over 5 years old (Intel Core i7-980x @ 3.33Ghz stock frequency with 3.46Ghz turbo clocks. Its been run like this at stock since new).

This never ever happens under heavy load its always a light load on the PC when this happens.

Please can anyone tell me what they think & or offer any suggestions to isolate the problem which sometimes does not happen for weeks then suddenly without warning it happens like earlier today & never leaves an event viewer entry either just the usual power was interputted restart error.

There is no pattern to this its always random I am going to have trouble buying another EX58 Socket 1366 CPU motherboard as they stopped making them about 3 years ago

 

[AWPC]

I know you said the event log is clean, but do you get a BSOD at all? If so, it would be worth using Who Crashed or Blue Screen Viewer in order to get more detail.

It's usually recommended to run memtest for a minimum of 8 passes, I've just been through this experience myself where it was fine for a couple of passes but would error later on.

No BSOD at all it just restarts without warning & only when the PC is not under load on the desktop not when its under heavy load gaming.

 

[AWPC]

That makes diagnosing the issue a pain

Probably not worth running driver verifier to rule out a bad driver if no BSOD.

Do you have a spare PSU you can try?

Yeah think I am going to have to do that next as this issue is so random that's the only way I can see trying to find the issue.

Thanks for your advice.

 

[AWPC]

Any chance you have an Asus Rampage 3 ?

My old one did that. Going back a few bios revisions fixed it. Althought that stopped the PSU was faulty a few months later anyway.

No its a Gigabyte EX58 UD5 rev 1 socket 1366 motherboard circa 2010 bought it brand new been ok apart from 2 Bios CMOS battery changes.

 

[AWPC]

RMA time its happening more often now so had to swap it out Seasonic can replace it now!