I forgot to check other vans that was on the lot, but still can't figure out why HD music cuts in and out in the van. Some days it works great, other days, no HD sound just standard stereo (you can notice that the highs notes are muted). And stations with multiple sub-channels, the sub-channels don't work.
The HD Radio cutting out is likely normal. It's a digital sub-carrier that's broadcast in the same frequency range as the analog FM (or AM) radio waves. Very much like digital HD television after the transition from analog to digital, it's either on or off, so to speak. Where analog radio can fade in and out and have some occasional static and background noise, when the broadcast signal drops out of a moment you'll drop out of HD receiver mode and should fall back to the base analog broadcast. This can be more problematic if you're listening to a sub-channels (i.e. listening to a different program on an HD1, HD2, etc. stream).
There's many things that can affect the signal. Location of the transmitter in relation to where you're driving (the digital radio broadcast range has a much smaller range than analog), atmospheric conditions (even more problematic on AM HD), driving by tall buildings or underpasses that temporary interrupt or degrade the reception.
I haven't listened to many HD stations since picking up my van recently, but I did notice a couple occasions when changing inputs or just after starting the van the head unit would lock onto an HD stream but have no sound until changing the channel.
HD Radio (a trademark of iBiquity Digital Corporation) is normally best for stationing reception such as home or if you live very close to the transmit tower, always have a strong, clean signal and reception path. There is some minor digital error correction built in to the broadcast but it takes very little for it to drop out and fall back to standard analog.