I believe I noticed something similar when we first got our FEH in November. Now we don't have the NAV system, and I was pretty certain that we don't have the automatic volume control, so I was a little baffled.

I eventually chalked this up to a crappy factory deck (had the basic 6 disc changer in the '05), and put in the deck I'd had in my civic, and have never had the problem again.

Maybe it was a surge in power, and maybe the factory deck just doesn't handle the surge effectively. All I know is the problem is gone, and I've traded 6 discs with some annoying 'features' for a single disc mp3 head unit that I'm used to.