new MBP problems with freeze ups

I went out and bought one of the new unibody aluminum macbook pros last month and I am very pleased with its performance so far except for one annoying problem that keeps happening.
It doesnt matter what I'm doing at the time, watching a movie, listening to music or just nothing. The speaker will suddenly emit this weird high pitched noise and then everything freezes up. The whole screen goes completely black except for the very top where theres some multi-colored lines and a part of my desktop thats still visible. Both the keyboard and mouse are unresponsive so all I can do is hold down the power button.
Ive never seen this happen on a mac before and im hoping theres a fix. Any help would be much appreciated

it seems to happen only when Im running the 9400M, when I switch to the 9600M GT it doesnt seem to happen at all, or maybe less frequently...not sure. I suppose it could be a driver problem?

also I have after market memory, would that matter?