Nokia :: 5800 / Paired With JVC KD-BT11 Car Stereo

I've found something I don't like about my 5800.. It fails to work properly with this head unit for A2DP.I've paired the two up, the head unit and phone show as connected, but when I go to play a music file on the phone, I hear audio for a few seconds and then it stops coming out of the speakers. The phone carries on playing it, and if I switch off the head unit so that the blue tooth disconnects, then the sound is heard through the phone speakers.I seem to recall that the N95 I have worked to play MP3. The only other significant difference I can think of here is that the music I'm playing is M4A.Any ideas why this doesn't work properly?