My iMac 24" is about 3 years old.
2.66ghz Intel Core 2 Duo / 4gb RAM / OS X 10.6.7
What happens is while I'm in the mid of using the machine, it suddenly goes back to the blue screen and then the login screen. So It's not a hard restart, more like a forced log out. This is the 3rd time it happened this week. It's never happened before. There was no warning, or slow down prior to the shut down. I also felt the top of the iMac to make sure it wasn't over-heating. It wasn't.
What's causing this and what can I do about it? It's extremely annoying to lose work due to this unexpected behavior.