I was having problems with my Radeon 6850 underscaning each boot (it wouldn't save underscan setting). Each time I rebooted the computer it would return to black bars and underscanned. I didn't find much even with a lot of searching.

Then I found http://www.rage3d.com/board/showthread.php?t=33991741

The solution was the optional step after step 9 in the guide:
(Optional 9b. if you have black bars normally, disable underscan (always after lightdm has stopped): sudo aticonfig --set-pcs-val=MCIL,DigitalHDTVDefaultUnderscan,0)

Posting it here in case anyone else has this problem.