I have some ideas,
I would like to see the schematic and your code configs, also part # of the snubber diode. I am thinking . . . Diode too slow . . . Not hooked up right . . . brown out PIC due to reverse EMF, remember the back voltage from that relay coil and motor is several hundred volts, also make sure the PIC is properly bypassed on it's power pins with .01uf and a 10uf, you may need to run separate power lines from the battery for the load and PIC circuits.