ML 350 radio fm volume fluctuates

My ML 350 radio FM setting is real loud when I start the car in the morning, then it lowers so much I can bearly hear the station. I have to turn the volume up all the way to hear the radio playing. It also will blast up loud again periodically and just go lower again while I am driving. The dealership said the radio was playing so they could not see a problem. The dealership suggested that I bring it to a radio man. Isn't that wonderful, why couldn't the dealer fix my problem? Very frustrating, and I will not buy another Mercedes.