Random Restarts, no BSOD.

[No Fun Sam]

My build is around 4 years old and for the last 6 months when I put it under load it will restart randomly. The timing isn't consistent as it could run for an hour or restart instantly. Since starting to troubleshoot I have replaced the mobo, GPU, SSD, and RAM. I have also tried it with a different power supply and processor. The problem still occurred in all of these cases. The only time I was able to get it to run stable was when I removed the GPU and ran it off of onboard graphics. I am also currently running it outside of the case. Anyone have any idea what might be causing this?

 

Edit: Should also mention that the temps for GPU haven't gone higher than around 80 and the CPU generally sits around 50 under load.

 

My parts are:

Motherboard : ASRock Z77 Extreme 4

PSU : Corsair HX750

Processor : Intel i7-2600K

GPU : Nvidia 950

RAM : 8 GB crucial DDR3

120GB Kingston SSD

3TB Seagate HDD