Thanks for all the replies
I did run extensive memory tests, so I think I can rule memory problems out.
I cleaned out the chassis of dust etc a while back and put some machine oil on the main fan axis so it now runs noiselessly.
I ran some cpu temperature tests a few days ago and nothing showed up as a problem.
I will look into a BIOS upgrade but I thought BIOS was only involved at bootup time. Some of the lockups have occurred at boot up but a lot of them are long after boot, so doesnt that rule out BIOS as a cause ?
Maybe I can find a utility to check the p/s as I dont have a spare. When the lockup occurs, the power is still on (or at least, the chassis LED's are on that indicate system power up).
I have got a spare video card - I could install that..
There was a power surge a year or so ago that burnt out the on-board LAN port. I had to install a separate LAN card to get going again. But if that had had a knock-on affect on the motherboard, surely the problem would be persistent not creeping/intermittent ?