If nothing else works for you you might try to edit your X config file with a command like this:
sudo kate /etc/X11/xorg.conf
Locate the line telling your screen dirver other then "vesa" you might try to change it to "vesa". Such as:
Section "Device"
Identifier "Blah blah"
Driver "ati" ->> change to "vesa"
BusID "PCI:X:Y:Z"