Had exactly the same problem with my iPhone 5. To be honest when I bought the car I was expecting a USB port for my music as you just cannot beat a physical "cabled" connection. When I found out it was bluetooth only I knew that it would be "glitchy".It has worked fairly well, but most songs do tend to get interrupted,when it loses its connection intermittently.
Just recently the Navigon unit refused to recognise that my iPhone5 was even in the car in terms of PLAYING my MUSIC. As it still connected perfectly for bluetooth handsfree telephone calls.
This carried on for 2 days. All I then did was to disconnect the phone from the unit.(Go into "Bluetooth" menu on your phone and for "car phone" -choose "forget this device". Then try reconnecting.(ie let it search for nearby bluetooth devices and it should pick up the Navigon unit again,then enter the pin code-0,0,0,0,0 default setting I think) It has been working fine since but I am not holding my breath!!
Wireless playback is too prone to interference from over head power lines and all the other signals out there. I am sure they will add a USB port in future.Seems to be a nice gap right by the 12v socket,near the gear stick, ready for that purpose.
New Garmin derived unit for newer Ups also. Hopefully will be more reliable but I do not think that will be of help to us who have already paid up! In some respects it is not quite fit for purpose in my opinion.
MM