Mathew_

Firstly this psu is quite low quality I would recommend swapping it for something decent, Corsair, BeQuiet, Bitfenix or something like that. If you need help choosing new psu let me know how much you can spend and where you from. 
Other thing is you didn’t mentioned any temps of the system, it may be that gpu is overheating and shutting down system, I know you mentioned changing thermal paste on gpu for new one so maybe you didn’t screw it properly so die and heatsink don’t make good contact. 
What you can try is in msi afterburner decrease power limit and see if this make any change, if it does most likely it’s psu fault.

Corsair and Bequiet are good, you can try a couple other things first. Make sure all drivers are up to date, do a DDU on the GPU and reinstall the drivers using Adrenalin 2020. If it is crashing during any graphically intensive loads then it's going to either be the GPU having driver issues, power issues, or it's bad. If you uninstall and reinstall the drivers fresh and it still happens your next option is replacing the PSU if even after the PSU is replaced it still happens then it's probably the GPU. You could also try testing the GPU in another PC and see if it has the same issues if it does the same thing in a separate computer then you will know the GPU is bad.