Multi-colored lines only on screen when booting 17" iMac Core Duo 1.83Ghz New to StackExchange/Ask Different. Been a lurker for a long time, this is my first question/discussion. I hope somebody might point me in the right direction. 

I was recently given a couple old iMacs that were going to be thrown away by a family member. The others all work fine after a format and memory upgrade. However, one of them does not. It is a 17" iMac Intel Core Duo 1.83Ghz. Here is a link to the exact model info.

On bootup, I get the normal Apple start sound, but the screen only displays multi-colored lines. Here is a picture. I pulled the plastic bezel off, and tried removing the screen panel, and reseating the screen cable. I also removed and reseated the memory, and tried resetting NVRAM, all with no change. I don't need to worry about losing any data on this system, I will format it and clean install OSX when I fix the screen.  

What should I try next to figure out what is causing the screen issue? 

Thanks in advance!
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.