PowerBook - Screen alignment flicker

Hi,

Ever since a security update to Tiger 2 days ago (not sure if it's related to this but it first occured on boot up afterwards) my Powerbook G4 screen has had a weird screen alignment issue that is quite hard to describe.

The best way I can find to describe it is if you mentally imagine the screen being split into quarters, when you move the mouse cursor from one to the other a strip of the screen image moves by one pixel. If you move the mouse from top half to bottom half the flicker is on a horixontal axis, and if you move the mouse from left to right it is on the vertical axis.

Does this make sense to anyone, and is there anything I can do to fix it? Does it sound like a hardware problem?

Thanks,
andy.