Almost certainly you have cracked solder joints on the GPU, Try squeezing just to the right of the trackpad (Not too hard though, as the hdd is under there!)

My 600Mhz 16vram died because of this, as did my 800mhz and 900mhz iBook G3's

Only problem is, it's rather hard to fix. A shim under the GPU might get you a week or two of life, But A GPU reflow is really what it needs (and even that might not fix it)