Alpine CDA-9857 Volume Fluctuations

I purchased this head unit about 18 months ago and installed it myself. Works great, never any problems. Over the last few months, however, when I am listening to XM the volume will suddenly dip down to a very quiet level. A few minutes later, and unpredictably, it will jump back to where it was. The volume "number" doesn't change if I check it by turning the knob, but the actual sound level is different. Doesn't happen on radio/CD/iPod. Nothing in the vehicle has changed, including settings on the head unit, in many months. This is installed in a 2002 VW Passat with Monsoon. Thanks in advance.