I found these from Gentoo Wiki
Disable framebuffer in kernelconfig, recompile, reboot! This happend as i upgraded from 8.28.8 to 8.32.5.
This error may also occur after receiving the EDID information: There is a known bug (http://ati.cchtml.com/show_bug.cgi?id=509) which has been solved in the 8.32.5. Also, just upgrading to 8.32.5 solved the problem.
My own Radeon X1600 512MB works fine with framebuffer enabled though.