David This is likely to be the 3rd party charger not allowing the device enough current to charge and make use of the application at the same time. The Navigation software uses more current as it uses more processor power to run. I would recommend replacing the in car charger with the Official HTC charger to see if this resolves the issue. Please also test the charge of the mains charger to see how long the device will hold charge for under normal operation to see if the battery is behaving ok. Here is a little HTC Tip too, Dial in the phones dialer as you were to make a call when connected or disconnected to the charge source *#*#4636#*#* to obtain the secret menu were you can see battery status and information. Please let me know if you are able to resolve this issue?Good HTC tip in there - dial *#*#4636#*#* to see lots of secret technical stuff!
Photo from the HTC.com website - I hope they don't mind me using it!