Sound system and Sat nav silence

[jetlag]

Hi
I wonder if anyone can help with this bemusing challenge.
BMW X5 4.4 2000.
Fire up sound system - all is well for about 5 to 10 minutes then no sound. ICE working as I can change tracks/stations etc but no sound. If I leave it on I continue to get nothing so if I turn the HI FI off (and keep driving) and wait maybe 30 minutes then all is fine again and the problem does not recur for the rest of the journey which has been 3 to 4 hours in the past.
If the Sat nav is on it takes the sound from that too. Also if Sat Nav is the only thing on the sound goes in exactly the same way as the HI FI.
What is bemusing is sometimes it does not happen at all which is frustrating when trying to get a dealer to recognise there is a problem at all!!

If I actually stop and switch the carr off and then return to the car 5 minutes later its normally fine again.

If anyone has had similar problems or indeed can throw any light on this challenge I would be most grateful.

Peter (Jetlag)