Screen Issue - not sure

Greetings,

The screen started acting strangely -- vertical lines down the screen, shaking then freezing.

After a few restarts, the desktop came up and I was able to run disk utility and the hard drive is fine.

I took the machine apart to check connections, and everything looked fine.

Now, I get a grey screen with the apple, appears to be starting up properly, then goes to a blue screen with the mouse, which works (no OS X startup).

I've reset the PMU and PRAM, with the same result. My reading leads me to believe its the graphics processor on the logic board, however I've tried the shim idea to no avail.

Since my symptoms are a bit different than what I've read, I just wanted to confer with the experts before I try to resolder the chip. ($400 for an antiquated logic board -- wow!)

Thanks for the advice!