Same card on ubuntu hardy using xorg-pushers repo for git versions. No such problem here.
There was a recent change to radeon to set GARTSize default to 32. Maybe intrepid didn't get this yet. If not, you might improve performance by putting this in your xorg.conf driver section:
Option "GARTSize" "32"
I've actually gone to "64", but "32" probably is all that's needed.