I think this may be due to the current being right on the lower limit.
Try turning phone onto standyby or off for car charging.
Not much help if using it as sat nav tho.
HTC One X (Vodafone UK) - ARHD 18.2 Stock kernel.
HTC Desire Z (Vodafone UK) - Donated to the father-in-law to play Words with Friends
Touch Pro 2 (Vodafone UK) - ROM: Psychosis (Home baked using OsKitchen)-now sent to mazuma mobile....
O2 XDA Serra - Home Cooked ROM- now sent to mazuma mobile....
O2 XDA Stellar - KaiserChef creation based on Mbarvian's Solace ROMs - now Deceased
Orange M3100 - now just gathering dust.....