I have two 24" iMacs. They are identically spec'd. 2.8Ghz C2D, 4GB RAM, no hard drives and ATI 2600 Pro. One boots and started to load off my external drive with Snow Leopard and then the screen flickers and shows lines and shuts down and then restarts. The other one you hear the chime and the screen stays black. Did the 2600 Pro have a high failure rate? That's the only thing I could think would be causing this. Also is it an easy fix or is replacing the main board the only way to fix it? Thanks for any info.