Yeah, I bought the HD tuner for my Pioneer AVIC headunit. It does go in and out. Because it is a digital signal you either have it or you don't unlike an analog signal where you get static if the signal is poor. I think part of the problem is that I read somewhere that the FCC requires digital radio signals to have much less power than analog.
Around my town, it comes and goes depending on where I am. When I go to Chicago, I get a pretty good signal the whole time. What really sucks is that my favorite local station is HD only so a lot of the time I can't even hear it.
A potential solution to your daughter's problem is to go into the settings for the radio. I bet there is a setting to turn the HD receiver off. Mine has a setting (I think called "blending") that controls how it switches back and forth from analog to digital. I can tell it not to do that - which is the part bothering her.