Hi - make sure the MCLR pin on the pic is tied high with a 4.7k resistor. Then make sure you have a bypass capacitor (eg .01uF) very close to the PIC between Vcc and Vss. Maybe you need a snubber circuit with a cap, a resistor, and maybe even an MOV to smooth spikes. Also, make sure that you have at least 10,000 uF for every amp that your power supply is delivering, assuming its a linear power supply. Above all, make sure you have a clean path to ground! If none of this works, disable the alternator to eliminate that noise as a culprit. How close is the LM34 to the ignition and/or the alternator? Are you using a pic with onboard A/D?? Ive done this same thing for motorcycles with success, so I hope this helps!