Milo, I didn't get it sorted "when it still had a warranty" because the machine was built up from bits, not all of them new, not all from the same supplier. Initially the system was falling over every few minutes and this was traced to a BIOS issue. I updated the BIOS and everything seemed to be good. The occasional rebooting didn't show for a while and I have never been able to determine what the cause is. I have a second, swapable, hard drive which has WinXp loaded. I have used this for some applications but not for long enough to see if the reboots occur while doing so. It may be a Ubuntu issue, I can't tell.
So far I have not lost data or work, it is just annoying.
Rans6.....