My 2013 GS HD (non-ML) does have the HD/non-HD switching, but only on weak stations, so I agree with Lexus that the problem is not in the car. When it does happen (not very often), I just change the station - there are tons of them here in NJ. If it's a station that you -really- want to listen to, you can eliminate the automatic switching by disabling HD.

 

The only HD problem I have at times with the ML is skipping but it depends on the station and signal. If it happens I just switch the HD off.