Comment 13 for bug 68291

There is a known bug in tdfx with a patch available:
https://bugs.freedesktop.org/show_bug.cgi?id=7149

To see if this is affecting you, check the gdm or kdm logs and search for an undefined symbol error realting to "getsecs":
/var/log/gdm/0.log
/var/log/kdm.log

On my systems I can use vesa but the display is corrupted at 24bpp.