Unfortunately, I don’t have a spare PSU to test. Nor the thermal paste. And yes, the fan is running, but when I examined it I found that one of the four screws was not secure.Tightened all the screws, and now it is working for about three hours. Don’t know if it will crash again, but I will just wait and see.
If it fails again, I will try with another PSU, and will also try to get a little thermal paste on the processor.
Thanks everyone for all your help. Much appreciated.