Someone may have suggested this before but I did not see it. Freeze your Mac. Get it good and cold. If the problem does not reoccur until it has warmed up, you have a heat problem. It really sounds like a heat problem.
Heat problems can cause vram and vprocessor malfunctions, cold soldier joints, and kernal panics because of RAM and CPU malfunctions.
Don't freeze your Mac on a humid day --> condensation.
Could also be an intermittent open. I had similar problems, but no kernal panic, with a bad DVI-VGA adapter. Just wiggle the cable to get the right colors.
Have you tried connecting to an external monitor? That would rule out/in your LCD screen and the ribbon cable that passes thru the hinge, and the connectors of same.
