Hi and welcome from me too :)
If the HDD diagnostics comes up with no problems I suspect, as you do, that it is a PSU problem.
I have seen these conditions before and the cause turned out to be a problem PSU.
The thing with PSUs is that often they do not die ..... but function below voltage on one of the rails or the voltage fluctuates.
The easiest way to test a PSU is to swap it for a known good one.
The replacement does not need to be the same wattage ( for testing ) and if the wattage is greater than the present one this is fine.
Let us know how you get on.