I don't have anything to add except that I have the exact same problem on my 2001 Wagon. It is very intermittent. The radio seems to lose power completely and then come back on a few seconds later. Once this starts, it happens repeatedly and then usually it shuts down altogether for a while.
I have sort of noticed that it coincides with hard braking - which lends credence to the loose connection theory. If I get around to tracking it down, I will surely post my findings but I may have to wait until the problem gets worse.
I also have the well-known speed sensor problem. With the AVC enabled, the radio volume surges and fades with the wildly swinging speedometer. I doubt the two problems are related but in combination, they make the sound system seem a little posessed.
-S