display issue

I've been using Trisquel for months and i am happy with it overall but
one thing that has been troublesome is a display issue.

The problem is that parts of the screen show little pieces of screen
from other workspaces or are repeated from further up the screen.

It is not there all the time and restarting the computer seems to help.
Resizing the screen also help and so does scrolling but it is present
more then 50% of the time and is quite distracting.

Any advice would be great-Patrick

P.S I am running an nvidia card and I am using two displays.