I have had a similar problem with my 15" 1.67Ghz Powerbook for a long time. You can see a screenshot here:
This was resolved by replacing the logic board, but as soon as it was out of warranty the problem occurred again, and I have noticed that when I tap the casing in the bottom-right area, the problem sometimes goes away. Also, this issue does not affect any seconday monitors.
I would be interested to know if you have managed to resolve this problem in any way