Prior to 2 weeks ago, everything was running fine.  Then, the system would start crashing either to desktop, freeze requiring hard reboot or just reboot on its own.  Eventually, it turned into freezing or rebooting on its own.  This would occur between 2-20 minutes from the the time I would start up a game.  Fallout 4, middle earth, thinking it may be related to steam some how, I moved all steam files to an external HDD, still crashed.  Ran Diablo 3, still crashed. To me it appeared to be some sort of power issue, thinking the PS was starting to go.  The system is less than 2 years old.
 
Was going to swap out with a new PS, but the one I bought was too big for the case and will be sending it back.  IN the meantime, I stress tested the vid card, no crashes.  Stress test the CPU, no crashes.  Stress test the drives, no crashes.  While CCleaner was harassing me to do it's thing, I let it.  Lo and behold, no crashes.........for over an hour.  But that's when it crashed.  So now the previous 2-20 minutes has turned into 1-2 hours before crashing, and in the same manner, typically just reboots.
 
My plan:  Get another power supply and vid card, try them one at a time and see if I can game for extended periods.  If neither fixes it I may just do a new build because I see no reason for this to be happening, unless it's something slowly starting to go and those are tough if not impossible to troubleshoot.
 
Thoughts?

Sajin
You didn't test your ram?

hmm, not specifically, but figured it would have fallen prey to the GPU and CPU tests along the way.  I'll check it...

So I ran the full memtest scan...twice.....over 3 hours.....zero issues.  Still not sure where to lean on this issue, other than it being a power draw from multiple sources in which the PS may need to be changed out or the MB power related circuity is going due to low quality components.  
 
Last night I ran D3 and that time it didn't make it the full hour for auto-reboot.
 
Something I didn't clarify earlier.....the computer runs fine for everything else, surfing, streaming, boring stuff has no effect.
 
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update:
 
Started D3 at 10:27 and it reboot/crash at 10:30....3 whole minutes.

so....after disabling about half dozen startup progs, I ran D3 again
 
1100-1103: 3 minutes again?
 
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stopwatch timed: 5:20?
 
No idea.

So bad graphics card?
 
I was having a similar issue with mine, i could run all independent stress test and everything was fine. When i would play destiny 2, it would randomly crash the game and hang up where i could hear the game but just see a black screen. It would still let me alt tab to desktop, but could not click on anything (task manager to end task or other windows) so i would just simply log out of windows and log back in.
 
Since i was overclocking my cpu at that time and trying to iron out the overvoltage problems my motherboard has, i thought it was the cpu causing the problems.
Then i turned on my ram and thought that was the problem, all the while overlooking my gpu.
 
The gpu came from a different build and i felt i already knew what that card should run normally. It did turn out to be the gpu overclock, but the fix was intresting.
All i had to do to make the card stable was turn the power limit back 1% from 120% to 119% and now i can maintain the overclock i was using with no crashes and can actually raise the overclock and still be stable.
 
Is your GPU totally fried now?