PC suddenly started turning off abruptly

[Nantes]

My rig:

-    Core i7 3770K
-    Cooler Master 212 Hyper X heatsink
-    GTX 970
-    ASUS P8P67M-PRO motherboard
-    PSU: Corsair Builder’s series CX 500 V2
-    Windows on Samsung 850 EVO SSD; has 2 HDDs as well
-    A Uninterruptible Power Supply (stops the computer from turning off when there are power outages)

 

WHAT IS HAPPENING (hereafter referred to as “the problem”): Last wednesday night was browsing the web when my computer suddenly turned off. Since then it will not stay on for more than 1h06m (the record thus far). The DRAM LED is now permanently on. I took it the next morning to my trusted technician, who worked on the machine for two days before I took it back to do more testing myself over the weekend.

 

The problem does NOT seem to be related to the following:

 

• Graphics card. I switched my GTX 970 for an old GTX 560 I had lying around and the problem continues.
• Power Supply Unit. It’s old (5 years) and a pretty bad model to begin with (it is rated Tier 5 in the PSU tier list), so it was the first thing I suspected. However, my technician switched that for his Corsair 700 W one and the problem continued.
• RAM. I ran 8 hours of Memtest with zero errors; more details about RAM below.
• UPS unit. I connected the computer directly to the wall socket and the problem persists.
• Overheating. Temperatures are fine as ever. Even with the “maximum heat” Prime95 setting, my processor does not exceed 75 celsius. I also haven’t noticed any sudden increases in fan loudness before the PC turns off. GPU temps are fine even after a full benchmark of Uningine Heaven.
• Component stress. In fact, oddly enough, component stress seems to be a protecting factor. The three times so far where it has lasted the longest before turning off (40 mins to 1h06m) were with heavy gaming. As I’m writing this, Prime95 “blend” testing is going strong at 2 hours, so it’s breaking the record right now! Conversely, when I leave the computer idle it will usually turn off in 5-15 mins. There is not enough data yet to affirm for sure that component stress helps, but we can at least conclude the problem is definitely not caused by overtaxing of the GPU or CPU.

 

THE FACTS:

 

• The motherboard and processor were bought used from the same guy. Despite being a K model, I never managed to do any overclocking with the processor without creating instability, so I believe I was conned by the seller, who likely knew the silicon was botched. Before the problem started I would get a total computer freeze up to 2 or 3 times a month. Other than that and the no-overclock issue, it has worked well until now.

 

• It can turn off at any stage, from two seconds after turning it on to an hour of VR gaming. Since it can turn off before BIOS, the problem doesn't seem to be Windows/software related.

 

• Changing the PSU didn’t fix the “DRAM led permanently on” issue.

 

• The technician mentioned the clips that bind the RAM to the slots are loose; the computer wouldn't send any video after I transported it to the tech's office, which was fixed by him just pressing on the RAM modules with his hand. However, if the turning off problem was caused by faulty RAM, I don't think it would have survived the 8 hours of Memtest. Also, if it the problem was being caused by the RAM dislodging itself, the computer wouldn’t turn on anymore (as the RAM wouldn’t magically realign itself).

 

• Tech claims he once catch the processor go to around 90 celsius moments before the computer turned off. I have been running Core Temp on logging mode (grabs temperature and load data every 10 seconds) and so far it has shown no abnormalities whatsoever in the last grab before each turn off. So perhaps he was seeing things (saw a 60 and thought it was 90).

 

MY THEORY is that either the processor (for reasons exposed above) or the motherboard (the DRAM LED being a clue of that) is dying. I’d like to know which one, though, so I can plan what replacement to buy. Do you guys have any further ideas for diagnosing this issue?
I know this was a wall of text, so thank you very much just for reaching this far.