Mmi Error Message "please Check Nav Dvd"

[Pogo1]

Does anyone know the solution to this navigation problem? I have a 2006 A6 Avant. After an hour or so of operation of the Nav, the MMI displays this message. After removing, cleaning and reinserting the DVD, the Nav will return to normal operation only to fail again after a period of operation. Also, if the Audi is turn off over night, the Nav will operate fine when started only to error out again after about an hour of operation. When this error message is displayed on the MMI screen, the NAV information continues to display properly on the driver information display (located between the tac and speedo).