Every once in a while (once a week maybe??) the volume in my stock radio will go down just a bit. Not enough to reach over and crank it back up, but almost. A few seconds later and it will go back up to where it was. Local stations, and car moving or not dosen't matter. It's been doing this for a few years. It has nothing to do with an electrical load either (bc my lights don't dim when the volume goes down). I am thinking it is a result of the antennea (sp?) "diversity"system doing it's thing.