I recently started having trouble 15" Mac Book Pro dual core display. I would see a color scrambled horizontal line *see image.  I began monitoring the temp and noticed it was getting very high around 150 degrees, took approx 15-30 minutes then the display artifacts would appear and at times the computer would lock up or screen would go black. Well it turns out after about a year of running it in dual monitor mode with a 21" monitor the video processor was overheating. Since I no longer run it with an external monitor and the problem has gone away.