PC restarting under heavy load

SirYar

Honorable
Nov 9, 2013
11
0
10,510
Hello,
Lets get some specs out of the way first:
CPU: Intel Core-i5 4670 (No OC)
GPU: AMD Sapphire R9 290 (Stock, No OC)
PSU: Corsair CX750
HDD: WD Caviar Blue 1TB, 7200RPM
Motherboard: Gigabyte B85M-H3D
OS: Windows 8.1/10/Manjaro Linux (Issues persists on all of them)

Recently (around 2 weeks ago) I started having a very rage inducing issue with my almost 2 year old PC: It reboots upon experiencing heavy load, specifically GPU load. The PC would look like it's shut down, but after a few seconds it boots up again.

I can't run Unigine Valley without it rebooting after a minute or so, and I can't even start Furmark as it reboots instantly as the stress test begins. It also reboots randomly when playing Dota 2.

I tried running the PC without my GPU (using the built in Intel HD Graphics) and it worked perfectly fine. Furmark ran for a good 30 mins with no stability issues.

This makes me believe the issue is with either the GPU or the PSU, specifically the latter. My GPU works perfectly fine before the restart, as in there is no performance issues, no artifacts, no signal losses or what not. This, along side many negative reviews I've recently read about the CX750, make me believe the problem is with it.

Have anyone else had similar issues and can advice on a solution? Is the problem the power supply? If so, any recommendations for a replacement?