Why Is My Mac iBook G4 Freezing After Being Idle For A Minute or Two?

I have had my Mac iBook G4 for 5 plus years and have been having a freezing problem lately. I've read about the logic board issues and it does not sound like that problem thankfully. If I am doing something on the computer and let it go idle for a minute or two it will freeze. It also does it randomly during Google chats. I am able to move the mouse and click on the desktop to get the force quit option up and it will quit once I tell it to. It's just annoying that I have to constantly force quit while doing something. Any ideas?