Last summer I left for extended holidays and left my laptop in my desk for about 3 months. It wasn't plugged in but I didn't remove the battery and I might even have left it sleeping. When I got back, I started it, plugged it in, and started using it as normal. After about 25 minutes, it abruptly reset. Subsequent attempts to start it resulted in the same behaviour with increasingly short times between boot and reset.
I didn't have time to look at it just then so I left it alone, with the battery out, for about a week or two. When I came back to it, it had the same behaviour, although the time between boot and reset was longer again and then gradually getting shorter (from ~10-15 minutes down to about 500ms in just a few iterations).
The processor doesn't seem to get hot, and the RAM isn't faulty (I checked it in another machine). It runs Ubuntu, although I don't think the OS has anything to do with this. It's an HP Pavillion dv6 with a 64-bit AMD Athlon X2 and an ATI Radeon (mobile, I think) graphics card. It was working fine and I hadn't done any overclocking.
How would I go about finding more about this failure, and eventually resolve it?