Thank you all for the advice.
I have actually swapped over memory sticks - I have a dual system that can accept SD RAM or DDR RAM, though not simultaneously. Still the problem occurred, but that was before I cleaned the processor up and, in the process, moved the processor sensor slightly away from the chip and have since had no problems (he said, desparately looking round for a piece of real wood to touch!). The overall effect seems to be that the processor is running cooler (the sink and fan were very clogged with dust) and/or the BIOS is believes this is the case. I wonder whether the motherboard might have been shutting itself down to protect the CPU?
I have also removed one of the two HDDs out of a removable caddy, in an effort to prevent it getting quite so warm
I am slowly reinstalling all my software, but will check the RAM as suggested by Mac the Knife, and also follow-up stickyb's suggestion. I will let you know how I get on.
Once again, thank you all for taking the time to post.
Regards
STH