I ran into the same problem as posted here Dashed borders of my window manager (XFce)
I now found a solution but am not able to aswer due to my newbie state.
I found out that the problem only exists on my Intel video laptops (nvidia was fine) and occured only with themes having window borders with more than 1px width.
My solution was to create a file /usr/share/X11/xorg.conf.d/20-intel.conf with the content :
Section "Device"
Identifier "Intel Graphics"
Driver "intel"
Option "AccelMethod" "uxa"
EndSection
This switches off sandy bridge acceleration and turns back to slower but reliable uxa acceleration.
