I noticed something similar in my car and thought I was losing a few speakers or something. I noticed the DSP light was on and pressed the button to turn it off. The dropping in and out stopped. DSP seems to have issues if the car is not getting a strong signal from the radio station.
2005 BMW 330i ZHP 6MT. Built 11-19-04. Bought 4-2012.
Stock: Nav, leather, PDC, Xenon
Added by me: OEM Alarm, OEM heated seats, OEM aux kit, OEM Bluetooth, intravee, M3 strut brace, BMW Performance Intake, Nappa leather arm rest, updated LCM (triple blink, euro hazards, trifecta lighting, 3 stage brakes), temp gauge