Could be variety of things....
Sounds like it's a hardware issue I think...
First thought is that there might be a loose connection somewhere inside....do you travel around with your powerbook such that something might have been jostled loose?
It could also be some other hardware issue since I assume that powerbook is more than a few years old...
Do you have a local Apple store or Mac retailer that you show it to them? or a pc repair shop?
They should be able to give you a diagnosis and then you can decide if you want to get it fixed...