Sounds like an output transistor warming up and failing if it happens on radio and cassette, CD or aux. May be easy to fix with simple soldering, but new radios are nice with bluetooth and ipod connectivity. hardest thing is to get one that matches the dash and the BMW antenna, I have had good success with Alpine and JVC, the tuner in Sony, Pioneer and all the cheap brands have worked out totally crap for me. Try to get one with simple to use controls, they design them to sell to people who think only of fiddling first, and driving second.