Screen turns black randomly, no VGA, PC becomes unresponsive

[Sephyrias]

OS: Windows 7
Motherboard: Gigabyte 78 LMT-S2
CPU: AMD fx 6300
GPU: Nvidia gtx 760 Ti
Power supply: Cooler Master 500W RS-500-ACAB-M3

The PC has been in use since several years, so far I only had to do minor upgrades and fixes over the years (fans, etc.). It happened out of nowhere, while I was watching youtube videos.
I'm confident that this is a hardware issue.

The main problem is, that the computer crashes without bluescreen multiple times over a few minutes in a seemingly random pattern, until it becomes unresponsible (even holding the power button does nothing then), with the monitor receiving no VGA signal anymore and I have to flick the power switch to turn it off and on again. This can happen at all times. While booting, during system repair, before the Windows loading screen, after, etc.

I managed to do a full system repair, driver updates, malwarebytes run, etc. at one point, when the system was stable for 30 minutes after the PC had been off for several days and after I swapped out the CMOS motherboard battery (because I bought a motherboard speaker and it gave me the signal for a CMOS error until I replaced the battery and strangely enough it only gave that signal, if the GPU was plugged in and connected).
The computer also couldn't show a screen without GPU inside at all, until I had swapped out said battery. Doing that fixed the speaker CMOS error and allowed me to run the PC regardless of whether or not there is a GPU inside, but it still crashes into permanent black screens no matter what.

No difference between hard drive in/out either (aside from the obvious "insert disk" error when booting without a hard drive, if it turns on with a VGA signal for once).
Other computers can run via the same wall socket without a problem.

That leaves only RAM, motherboard (unlikely since the speaker detects nothing?), CPU, power supply, cables. What do you think is the most likely problem source? What can I do to narrow down the options further?

Update: Tried a new PSU, didn't change a thing.
Update2: Also turns black while I'm in BIOS, but I can confirm that the CPU temp is normal.

[_I_]

cm power supply
they have a few good ones but are overpriced, their low end is crap

[chiefputsilao✖️ping]

psu and/or gpu.

[Sephyrias]

Ursprungligen skrivet av _I_:

cm power supply
they have a few good ones but are overpriced, their low end is crap

Hm, replacing that is gonna take work. Well, at least not as much as replacing the motherboard I hope. It was my thought as well, since this does seem to be a power-related issue.
Got any idea what kind of power supply unit would be best for this set? EVGA 650 BQ, 650W? More? Less?

[Sephyrias]

Ursprungligen skrivet av chiefputsilao✖️ping:

psu and/or gpu.

I forgot to mention: I did get the chance to try another GPU at one point, changed nothing. Same behaviour.

[chiefputsilao✖️ping]

Ursprungligen skrivet av Sephyrias:

Ursprungligen skrivet av chiefputsilao✖️ping:

psu and/or gpu.

I forgot to mention: I did get the chance to try another GPU at one point, changed nothing. Same behaviour.

then get/use a spare quality psu like seasonic M12II 620W (modular) to rule out failing psu.

[Komrade]

Ursprungligen skrivet av Sephyrias:

Ursprungligen skrivet av _I_:

cm power supply
they have a few good ones but are overpriced, their low end is crap

Hm, replacing that is gonna take work. Well, at least not as much as replacing the motherboard I hope. It was my thought as well, since this does seem to be a power-related issue.
Got any idea what kind of power supply unit would be best for this set? EVGA 650 BQ, 650W? More? Less?

650 is overkill, you only need 550 watts max.

[_I_]

psu replacements are easy
unplug the old one from drives, fans, gpu, mobo
unscrew and remove
then install the new one

its one of the last things i install when building a pc

if you have a vga display, you can try with the onboad amd/ati hd3000

i feel sad for the mobo, its too weak for 95-125w cpus, only capable due to vrm throttling
760g chipset was not designed for fx, but modded to run them

the seasonic 550-650w psus are good and cheap, often on sale for $40-50
https://pcpartpicker.com/products/power-supply/#m=11,14,71&e=4,2&A=550000000000,2000000000000&sort=price&xcx=0

[Sephyrias]

Ursprungligen skrivet av _I_:

i feel sad for the mobo, its too weak for 95-125w cpus, only capable due to vrm throttling
760g chipset was not designed for fx, but modded to run them

What do you mean by that? My current motherboard shouldn't be able to run the AMD processor?

[_I_]

its a weak board, not designed for fx, but low tdp athlon2/phenom2 cpus

it can run fx, but not at their rated spec, and with the weak vrm config it will throttle at any load

[Sephyrias]

I guess I'll put a new motherboard on the "to buy list" for upcoming years then.

[Magma Dragoon]

Ursprungligen skrivet av Sephyrias:

I guess I'll put a new motherboard on the "to buy list" for upcoming years then.

Just dump the platform and get on Zen

[Sephyrias]

Update
I got a new PSU and installed it. Was quite a bit of work since the previous PSU was non-modular.
Same problem as before. It's not the PSU. It's not the hard drive. It's not the GPU.
That leaves only motherboard, RAM, or CPU.
Sucks even more because the cables that came with the new PSU don't fit into the casing due to rigid parts.

[[☥] - CJ -]

DDU in safe mode, install latest NVIDIA driver, Driver Only, Custom Install, deselect everything but PhysX.

If this doesnt fix it then its an OS or RAM issue.

Are you sure CPU temps arent the cause? FX CPUs suck when it comes to temps.

[Sephyrias]

Ursprungligen skrivet av ☥ - CJ - 8/6/19 - Meja 8/30/19:

DDU in safe mode, install latest NVIDIA driver, Driver Only, Custom Install, deselect everything but PhysX.

If this doesnt fix it then its an OS or RAM issue.

Are you sure CPU temps arent the cause? FX CPUs suck when it comes to temps.

Did you read the opening post? The PC also crashes into No-VGA-Signal state even without the Nvidia GPU plugged in, when it runs via the on-board GPU of the motherboard. I also kept track of the CPU temperature a few times when I could and it wasn't an issue.
I also once tried a different GPU, same story.
And yes, I did update the Nvidia drivers at one point when I managed to reach the windows desktop screen. However it usually already crashes when booting, even during the system repair screen.

[[☥] - CJ -]

Updating the drivers isnt the same as using DDU.

So this goes back to CPU temps and the OS
Are you able to get into the BIOS at all to see what the temps are?

Have you attempted formatting the HDD and reinstalling Windows?

Also, as you changed the CMOS battery going into the BIOS and factory defaulting it should also be done if possible.