I'm pretty good at making simple suggestions, so here's another one: does your right mouse button work correctly? I know it sounds dumb, but I had the same problem once a long time ago. I later found out that from all the plugging in & out the mouse connector got a bit worn out, so the pins didn't make good contact anymore..
Other than that: I have no idea, you may try reseating the kickstart chip but I doubt that that is the problem...
