First check to see if the video RAM slots are filled. Two internal Video RAM slots can take either two 256 kB 80 ns 68-pin VRAM SIMMs, or two 512 kB SIMMs. Installing one 512 kB and one 256 kB VRAM SIMM garbles the display. See if you can find, then press the reset button on the logic board. Did you replace the 3.6 volt PRAM battery? Next try zapping the PRAM (command - option and P and R keys held down on start up.)