Here's a perfect example of AM "HD" reception on a car stereo that keeps flipping back and forth between analog and digital -- with an annoying phasing/echo effect as it does so:
http://www.youtube.com/watch?v=UtPIK3JM4C8
You'd think the receiver would incorporate a counter that would keep track of the number of HD dropouts in a given time, and if the dropouts become too frequent, it would force the radio into analog mode until a higher threshold of HD signal quality was reached -- but I guess not!