i just posted this on apple support, but i think it might help to have here as well:
right now i havea an ibook g3 with strange screen issues. the owner who asked if i could repair it says that it fell off his desk with an ethernet cable plugged in. so the ethernet port is completely crushed (the trackpad casing is visibly dented when you take off the keyboard), but this also started screen issues. the computer can start up, but tapping it or dropping it even an inch on one side screws up the screen. it just starts to scan lines down, and it will change if i firmly tap the left wrist rest right by the keyboard. the computer completely freezes when this happens too.
since the screen does this and the pressure point is near the video out port, i suspect something in the display system broke or shorted. however, rebooting leads to a blank screen, more lines, or a perfectly normal computer. so might the hard drive be at fault as well? it was running when it dropped, so there seems to be a number of things that could be damaged.
but just now, i tried booting the ibook again the the hard drive sounds like it's trying to spin but can't. a very sad sound, really. that might be the new primary cause.