My rMBP has, since day one, disliked external monitors. After the OS loads and I log in, its all fine, but boot up and log in resolution freaks out every now and then. There are black bars on the top and bottom, and everything is a pain to read. This can start sporadically, but it will always start if I boot with the HDMI port in use. It wont go away until I reset the PRAM. Today, I had to reboot because bluetooth kept disconnecting. The graphics were screwed up again. A PRAM reset fixed both. Why do I need to reset the PRAM so frequently?