Multi-colored lines only on screen when booting 17" iMac Core Duo 1.83Ghz
Probably a hardware issue either with the GPU or something between the GPU and the screen (screen itself, cable, etc.) dying. I know you reseated the screen and cable, but if said components are failing/have failed that won't be enough.

If you have an external monitor, try plugging it in and see if it works. If so, run Apple Hardware Test and see what it reports back. If you GPU is indeed dying, chances are this won't work. To my knowledge the GPU is attached permanently to the logic board, so you can't replace it.