I have the same problem for the last few months (red pixels where black should be / comes and goes). I found out that if I have a usb stick plugged in and I move it the problem goes for some time. After that I realised that pushing on the metal case near the speaker would have the same effect.
From Apple they suggested that I should replace the logic board but then again they suggested that I should replace the screen as well on a different appointment, so..
Any suggestions ?