IPhone X Volume Problem in Car Only

[Kaner1]

When using Spotify in my 2015 Subaru Outback, the volume randomly lowers then goes back to normal.  Seems to last anywhere from 3 to 6 minutes.  I do not believe it is a function of my speed (lowering when slowing down, etc.).  The volume does not do this when I play through my Iphone or any other device - portable bluetooth speaker.  It is only when I play through my car speakers.  When speaking on the phone in the car the volume is fine. Just started happening about 3 months ago.  Any help would be appreciated?   Thanks.