What mode are you using when it prompts from a USB connect? if it doesn't prompt you, you should be fine, or otherwise the car charger is defective if it goes on and off
I use the Kensington usb charger since it was only $10 and it charges anything with usb
while this one is much newer than mine, i highly recommend this one, i've had mine for about 5 years now and no issues
http://us.kensington.com/html/17524.html
