What I think of immediately is that the monitor gets out of sync and that's underpinned by the fact that you use a DMI to VGA adapter
use the Xfix option from recovery menu....
At every boot?
Please post the content of /etc/X11/xorg.conf
What you can do to temporarily help out is to edit that file
It's not the best graphics but it works - but you have to turn off Compiz - visual effects to minimum
A permanent solution may be:
Read the specs for your monitor and find out the horizontal and vertical frequencies
HorizSync 30.0 - 81.0
VertRefresh 56.0 - 75.0
I have the correct values above
Driver should be nvidia
If you have "nv" now that could be the explanation as some cards and/or monitors don't like "nv"