Sounds like an unusual problem. If everything works correctly except for this issue I would expect there is a problem with the display unit, but it could also be a problem with the controller. Be sure to do an exhaustive check of all connections just to make sure.
The good thing is that you can still use the system while figuring it out. The cheapest and easiest test would be trying another display unit. If you have access to one, maybe even a nearby ES member you could visit would allow you to plug his display into your system for testing. Otherwise you might use this as an opportunity to build up your parts bin, maybe even get another model of display that you might like better.
If a different display works the same, then it's probably a controller problem and you will have to decide if it's worth fixing since it is usable as is.
Be sure to check with your dealer if its a fairly new unit.