Norman, the computer spec link you mentioned is similar to mine. I have done a D-A-L hardware scan and have attched it to this message.
I ran SPEEDFAN and have attached screenshots. I am not reallt sure what LOCAL and REMOTE mean, but the REMOTE temperature was considerably higher yesterday.
Interestingly, today being cooler, I have not had any problems.
I am pretty sure that the problem is temp-related. Maybe I'll try re-compounding the heatsinks.
I am still wondering if I should go the graphics route or the PSU route.
I now tend to think it's graphics and the problem is temperature-related.
I'll maybe try and sort out the heatsink, or install an extra fan.
Failing that, could someone suggest a garphics card. I am not a gamer, so I really want something of similar spec to the on-board graphics I have.
Also, I tried Prime95, but was unsure how to use the program.

I have just installed an extra fan directed toward the motherboard and I'll see if this cures the problem.
By the way, does anyone know if my PSU fan should be operating continuously ?
It never seems to be coming on.

Do you have both on board graphics and a graphics card? If so why don't you try disabling the on board in the bios and using the card. Are you still having your initial problem?

Prime95 has to be installed and then run from the start menu. Then you just leave it sitting there.