A quick Google returns this interesting link:
http://forums.amd.com/game/messageview.cfm?catid=440&threadid=168568&messid=1324412&parentid=1322139&FTVAR_FORUMVIEWTMP=Branch
indicating even the Windows AMD driver has trouble.
Have you tried the vesa driver as a test?
Wow! I didn't come across that link during my research, but it's
bloody amazing that this is a cross-platform bug facing all
combinations of AMD video cards using the S27C500 monitor. I will give
things a try using the vesa driver, thank you very much for pointing
me in the direction of that link. I believe I'll be going to the store
to buy an Nvidia card later today.
I am happy that this is a real bug on AMD's part, and I am angry they
have turned a blind eye to this problem!