Comment 6 for bug 455601

"man exa" list some different EXA options that might help for the flash problem. But if the original issue here is still present with XAA, those options will not help.

I guess Option "NoAccel" "true" will fix it but it will be unbearable slow.

I suspect you're running out of video memory. Try reducing the depth to 16, or specify a virtual display size just as large as your resolution (see man xorg.conf).