Damn, I just sent my wife out on a 6 hour trip and set her phone up for the trip by pairing it to the car (BT). As soon as she launched Nav, the phone kept rebooting. She had to get her friends standalone nav unit so they could navigate. I thought maybe it was too hard on the phone to be streaming Pandora while navigating, but it would crash (do a soft reboot back to the boot animation, not the HTC logo) as soon as I opened maps and started navigating. Then it would mess up the BT radio as it wouldn't toggle on/off and froze at "turning on", so I had to do a hard restart to get BT to work again.
It's awesome that as soon as she leaves, I come here to see if someone has posted this issue and it's the top thread! lol. I called and told her I think I can fix it for her next time she stops and I can talk her through it. So I just have to tell her to go to settings > apps > Maps > uninstall updates (just going off my memory for the settings), then she'll be good, right? She's totally stock/unrooted with the ICS update.
Thanks!