The first thing that comes to mind is enabling EXA acceleration since XAA is the default on some chips. In the radeonhd driver section of xorg.conf:
Option "AccelMethod" "exa"
Not sure about this, but I *think* that AtomBIOS support is not on by default, in which case you can try if AtomBIOS works better for you with:
Option "UseAtomBIOS" "true"
Btw, what Radeon card are you using?