have a 2005 cayanne and having problems with audio/nav system cutting out on starting vehicle, screen goes black then reloads with porsche sign then a message that it's not working which I have to press button to confrim, at this stage the radio and single cd player will work but no cd changer or navigation system, please help!!!

Press on/off button on stereo with key out, everything works,tem when key in everything still ok but when engine turned on everything goes off and comes back on with only radio and single cd working also error sign lights up that cd/nav system is off.