My PC is behaving quite strangely

[THINKPAD_Fighter]

Hi all, 

I built my PC two months ago, and everything was going great.

Here are the exact specs : 

Intel Core i3 12100

Corsair Vengeance 3200MHz 2*8Gb 

Corsair CV550 550W PSU

Gigabyte B760M X AX DDR4

GeForce GTX 1080

Here's the problem : 
That very same GTX 1080 was bought from a friend who upgraded to a 4070
Card was working awesome, but running a bit hot
Took it out, repasted it, temps are now fixed, PC is running in tip top shape
Played 2 hours of CS2 with no issues 

I took it back out this morning to cable manage, and, here's where the problems started 

I put my GPU back in, couldn't get the pc to post again, it turned on, all fans spin, but no power light on the case and no post. 
Took the GPU back out, played around with RAM, made sure all connections were secured, and reset CMOS
Alas, the PC turned on 
But, it now RANDOMLY resets, sometimes 1 time sometimes 3 times, get just fan spin, no post, no LEDs, or just won't do anything, even fans won't spin

It doesn't happen with a pattern, when it does this, it could be on the lock screen, or running Prime 95 and Furmark 
The longest I got was 18 minutes out of it, while running prime 95 and furmark. 
I really don't know where this could be coming from 
Thanks In advance for y'alls help 
Sorry if this was long, I tried to inculde as many details as I could.