My A1212 MacBook Pro just started doing this same thing this afternoon. I reseated both sides of the video cable to no avail. I ended up pulling the logic board and reflowing the X1600 chip. That took care of it. It started acting up while watching a streaming video, so "heatey"-come, "heatey"-go. Sorry about that pun, I couldn't resist. Oh no, a double pun!