A/D converter problem

[netstranger.nz]

Hi Everyone,
I am having difficulty using Pic’s onboard A/D converter. Can someone give an advise please.
I want to measure the voltage across automotive battery when the car is running. When the vehicle is stationary the power to the PIC is off (to prolong the battery quiescent time). So I assembled a simple test circuit on the bench. The problem is that when the power to the Pic is off it still could be powered if the potentiometer is in high(5v) position. But the one that worries me the most is if the potentiometer is in midway( I read around 1.3v at Vdd with Pic’s 5v disconnected) position the Pic locks up and I have to reconnect the power several times to reset it and make it go again.
I really don’t want to use external brown out circuitry if possible. May be you’ve come across the same problem. If yes please share your experience.