My computer is crashing very frequently, spontaneous restarts, (freezing before) blue...

Problem: My computer is crashing very frequently, spontaneous restarts, (freezing before) blue screen with and the screen turning different staticky colours (and so no specific error codes).

I thought the reason was because of extreamly high temperatures from my graphics card (could reach 91c) so I bought a new case with more fans, the temperature seems to be better however the crashing and other problems became worse. Everytime I try to open a game the pc just crashes or restarts now while before it was very occasional. I'm not sure how to diagnose it but from what I know I think it might be an issue with the power supply or motherboard.
Pc specs:
PSU: RM 850E
GPU: 980 ti

CPU: 4790k
Ram: 16gb ddr3
HD: 1tb

SSD: 120gb

Case: montech king 95 pro

Cpu cooler: 120 mm aio
Windows 10

:)