Put a 1n914 in series with battery + to PIC Vdd and a 1uf tantalum from
PIC Vdd and diode cathode to ground.  PIC is protected from reverse
polarity plus noise and spikes on unfiltered side of diode.  Works great
with transistor driven inductive loads of 1 amp with below 3 volt supply
if you you don't source any curent from the PIC.  It's the negative
spikes from the PIC or transistors driven by the PIC sinking current
that cause the problem.  Especially as voltage drops and internal
resistance increases in the battery.  Isolate the PIC.

Don