Hi!
I had the same issue. Freezing, scary gfx - failures,.... Anytime I tried to use my external projector these problems occurred.
I tried everything I could possibly think of (Reboot, PRAM Reset, Use only one gfx - card, etc...).
The simple solution: Switching "off" the translucency in the accessibility section.
Everything works fine now. And I even allowed the automatic switching of gfx - cards. Still stable.
But to be clear: I want a system that just works. I do not want to work hard to get my rMBP (2012) ready to use. I could by windows if I wanted to...
Apple fix this! 😠