Hi chaps,
Thanks for all the good advice. It could be a PSU problem I've replaced PSUs in the past okay, I'll have to hold off until well after Christmas to replace it if that is the problem. I'll also have a look at the CPU cooling fan replacement side of things although it could be a real PITA to fit. I tried the War Thunder open beta yesterday evening, was actually enjoying playing a game after so long, oh the irony!

and it crashed out twice when the PC shut down, have the same problem with the X_COM demo. I rolled back my Nvidia drivers just in case the latest ones were causing problems but it didn't resolve the issue.
Cheers and thanks
Aces