Strange behavior from a PB G4 (Part 2)

On to my next issue. Often when I'm typing the insertion point will jump to someplace other than where it should be or just disappear all together. I'm not doing anything to cause it to do this. I've been being really carefull to not to do anything wrong. I'ts often enough to be a real nuisance. It happened 7 times during my first post (Strange behavior from a PB G4) and 3 times during this one. Please help. Thanks.