I had the same problem and after a while I figured out what was causing it.
The unit has a "feature" where if you touch a spot on the map, it "marks" that spot with a little almost invisible red circle and then the map stops scrolling. I presume this feature has something to do with marking a spot so you can do a POI search around it, or to save it as a favorite spot.
Anyway, the only way I have found to get it out of this special mode is to simply press the map button. (the physical "Map" button on the top left of the unit, not a touchscreen button)
Hope this helps.
