Help! PC Reboot

[genesis25]

Specs:
GPU: AMD R9 280X tri - x 3G D5
PSU: Cooler master semi-mod G750M
RAM: Corsair vengeance pro 2x8 gbs 1866mhz
CPU: i5-4690 3.50ghz
Storage: 1xcorsair force 128gb SSD , 1xtoshiba 1tb HDD
MOBO: Fatal1ty H97

Problem: Games has been running fine on this PC until a month ago, PC reboots (something like if you hold down the power button) when CSGO and LoL starts a game, dosent reboot at the "lobby".

I have tried prime95 on blend and pc remains fine but it instantaneously reboots when i ran furmark on normal settings.

So does the problem lies on my PSU or my GPU?

Thanks in advance!!!

 

[jtjt00]

Furmark stress the GPU, but PSU also play a part too during the stressing.