Can you get MSI afterburner and take a look at the GPU clock graph, and see how it varies over time when playing the game.
MSI Afterbuner:
https://gaming.msi.com/features/afterburner
A note on your PSU, it is of pretty low quality. On the tier list, from tier 1 to tier 5. 1 being the best and 5 being unsafe, yours is a tier 4 and goes along with the description
'Built down to a low price. Not exactly the most stable units ever created. Very basic safety circuitry or even thin gauge wiring used. Not for gaming rigs or overclocking systems of any kind. Avoid unless your budget dictates your choice.'
It is usually recommended to get a Tier 1 or 2, with 3 being okay. This could be the cause of your issue, but before it is put down to hardware, there are a few more things to try. By chance, you don't have another PSU lying around?