I seemed to have fixed the problem for the meantime, but the PC still acts up. Same problems as stated in the original post. Now a couple of the fans won't spin up from time to time, they do light up though.
Unplugging all the fans and lights and starting up the PC solved the problem temporarily. I plugged the fans in 1 by 1 after it booted fine. There's also no beeps or anything, just the noise from fans spinning.
Everything is properly plugged in and seated firmly. I don't have any spare parts on me. Are you guys sure its my GPU and not the PSU?