Flickering secondary monitor since the Shuttle update.

Details: a laptop plus an external monitor. It goes black for a moment, comes back, sometimes teh windows jump to teh main screen :D Ive rebooted plenty. Id love a checklist rather than random tips, since Ive already tried random tips.. 馃敟 i have about an hour tonight to test things, and i would rather go through them in a sensible order. Tell me what u would check first and i will report back with results. pls help 馃檹