Hi there,
I have a full hd monitor and 5770 ( plugged in HDMI1.4) as well and i noticed 11.10,11.11 drivers crashs when i'm modifing any options related to HD and or hdmi settings . I went back to 11.9 and it's working better now even if it still crashs . I have no idea why...
Not to tell that 11.9 recognise native 1080p native monitor HD when 11.10,11.11 not ...
So maybe you should go back 11.9 and test it .