Enter the GRUB menu at bootup and select the rescue mode. This will take you to the command line prompt.
Then edit your X11 config file below and change your horizontal & vertical sync. I usually add +/- 20 on both and reload. If its a H/V sync range problem then this will probably fix it.
* edit H/V Sync +/- 20 to the existing #'s to increase the range
* then to save it type ":wq" without the quotes and reload.