I have the same vehicle you do (Charles). I just bought it last week, not from dealer. The audio mode switches from SAT over to AM on its own without me even touching anything, sometimes it even cycles through AM, FM, etc.
Being my first Lexus, I am VERY glad to know it's not just me and I am NOT crazy. This is incredibly frustrating.
Just a few minutes ago, I was sitting a stop light not touching ANYTHING and it starts the little routine again!
I guess I will have to take it back to the "pre-owned" dealer where I got it and see what they will do.......great.
Now I'm wondering if the nav system is screwed up too, it doesn't seem to perform correctly either...
Personally, I'm starting to feel like this just has too many unnecessary features that are incredibly expensive when they malfunction.....