Comment 67 for bug 565883

Based on the description of the symptoms, it sort of sounds like this might be caused by an incorrect AGPMode setting, which is relevant when DRI is enabled. You should test if changing this makes the issue go away.

For KMS configurations, agpmode is a kernel module parameter. Enter the grub interface during boot and append radeon.agpmode=X to the kernel boot line, where X = -1,1,2,4,8

1-8 are the agp modes,
-1 uses the onboard gpu gart (PCI, PCIE, etc.)

For details about this with UMS (non-KMS), please see the "ATI AGP Mode Quirk" section at https://wiki.ubuntu.com/X/Quirks