Same here - I bought my 2006 S-60 with HU-650 used (about 70,000 miles) and it always had a ghost that played with the volume - not that often, but obviously wrong. I added a XM radio and that works fine, up until a month ago. Now the evil spirit rolls through the AM/FM/CD sometimes - can't pin it to anything I'm doing. I just set the 3 FM stations to the frequency I need for the satellite, and I have a 3 out of 6 chance that when it rolls, it will settle on a channel I can use. (yeah, and 11 out of 10 times, toast lands jelly-side-down!)
Glad to hear I'm not alone - anyone try replacing the 650 with a replacement?