I now have fully stripped and rebuilt the motherboard, cleaned and dusted everything. There was quite a lot of dust and I considered opening the video card up to clean but couldn't get the cover off.
Anyway, all back together and running fine, no crashes since for around 24 hours now. Forcing many different types of restarts. Let's hope that it was just that. Interestingly, I found that the power on/off switch wire and the reset button were both connected to plugs on the compro TV card and not the motherboard. and even more surprising was a sticker on the TV card that reads, 'turn power to pc off for 30 seconds should computer suffer from constant restarts'. Very suspicious. We will see how it performs without this pci card for a while.
Thanks again for all your help, I will give it a couple of days, if all good, then I will mark this as solved.