It could be several things
1. the screen itself is starting to flake out (I think this is the least likely cause)
2. the nav drive (where you load the map CD) is going bad (most likely cause)
3. the wiring or connections to the screen or nav drive are loose (not likely, but easiest to check and fix if a connector is simply loose).
If you have someone near you with a Mk3 or Mk3 nav drive then I would suggest swapping units to see if the the trouble follows or clears.
and :xoutpost: