Thanks folks, I have been running ASUS Probe to look at temperatures etc.
Right now PC has been running uninterrupted for 48 hours and CPU temperature has ranged between 35C/95F when idling and 38C/100F when in use. Case temperature typically a couple of degrees lower.
CPU fan has been running at a consistent 2550 to 2575 RPM throughout.
Power supply is most variable with +12v anywhere between +12.2v and 13.2v, +5v anywhere between +4.7v and +5.4v etc. The history graph for voltage looks like a scatter diagram rather than a line.
Yes I can take side off case, and I know it's dusty inside - particularly the CPU fan. I'll get a can of compressed air over the weekend and get it cleaned up.
Agree CPU overheat most likely, but can't reconcile that with failure only ever within the first 5 minutes after a cold boot! But will get cleaned up and post back next week!
Cheers
RC
PS ... got rid of that dumprep process, thanks E-Liam.