Question PC randomly shutting down

Dec 9, 2020
Lately the power on my home was going out in short bursts and my PC was turning off, so I bought a UPS and thought that would solve the problem, however I now have the UPS and it is working correctly (I tested it) but the PC still randomly shuts down without any notice nor blue screens, so I'm guessing maybe its a PSU problem. So I opened the PC and tested each pin of the PSU's 24pin ATX connector and peripheral connectors with a multimeter and everything seems fine, all are ±5% of the nominal voltage, its a Corsair RM650 low noise PSU. Another probable cause could be overheating, I used AI suite to test the temps and the CPU its indeed on the hot side (40 °C idle - 80 °C load), but even with the load testing the pc didn't shutdown, so I'm not sure... I really don't know what could be causing this problem, any help would be greatly appreciated. Thanks!

I ran MemTest86 and it seems there's no errors on the RAM, so that's one cause less I guess, I'm gonna replace the thermal paste to see if its the overheating whats causing the problem...

Update 2:
I changed the thermal paste and now the CPU stays at 30°C idle and 60°C load, but nevertheless it keeps shutting down without any notice randomly after like 30min of being turned on... So I guess that rules out the CPU overheating.

Update 3:
I removed the graphics card and tried using the PC like that and so far it has not shutdown, however I'm not really sure if now the PSU doesn't fail because it has a lower power draw, or the graphics card is the culprit...

Case: NZXT Phantom 410 Mid Tower
Motherboard: Asus Sabertooth Z87 LGA1150
CPU: Intel i5-4670K LGA1150
Cooling: Default OEM CPU Fan
Memory: 32gb (2x16gb) Corsair Dominator Platinum DDR3 1866MHz
GPU: EVGA GeForce GTX 660 SC 2Gb GDDR5
PSU: Corsair RM Series 650W 80-Plus Gold Certified Fully Modular
HDD: Western Digital Blue 1Tb 7200RPM

I built the PC back in 2015, it has never had any problems.
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