This is an odd problem that I have never heard of before. I would recommend that you ensure you have the most current software in your unit. If you do, please backup your data to a SD card. (MENU - FILES - Export User settings.... then export your points and routes)
Once you have backed up any needed settings/data, you should conduct a factory reset of the unit. (HOLD CANCEL when powering up the unit, until it enters the INSTALLATION WIZARD) This will take about 3 minutes.
After setting up the unit and restoring your files, if it still has a problem then it should be COMBO updated with a SD card.