What color is the screen when you boot it up? Some people don't know, but the colors displayed during the boot are the system testing its component. Example, if the screen turns red while booting, you've got a kickstart rom problem. That could help you troubleshoot it.
Look at the PLCCs... if any don't look well socketed, remove them (and while you're there, might as well clean their pins and socket with alcohol) and reseat them.