I've had a similar problem with a PIC controlling a ticket
dispenser. The cause was a string of 1us pulses reaching
Mclr as the motor turned. The solution was to isolate the PIC
from the motor's supply using a 100 ohm resistor with 470uF
+ 22n cap next to the PIC and decreasing the Mclr pull-up
to 1k as well as increasing the Mclr cap to ground to 1uF

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