If your computer ever randomly dies and restarts, run a memory test.
I am currently trying to fix a neighbors desktop that does just that, and my dad suggested trying a RAM test. It was trivial because Ubuntu gives you a memory test on their install discs so I dropped on in and ran it. 50% in it started picking up errors every few seconds, which would explain why it was acting oddly. It's like your post-it notes disappearing when you randomly decide to stick them to one certain place on the wall.
The first thing you try when RAM starts giving errors is to pop the SIMMs out, blow out some of the dust, and reseat them. It will usually fix it unless the RAM is really defective. Then you're SOL.