Since you're dealing with 12V, why not use a MOSFET switch? It's probably more economical and provides longevity and no EMI.
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Also you probably need a 0.1uf decoupling capacitor near the uC power pins to block motor start transients. If there are bad transients resetting the micro, place a 0.01u, 0.1u and 1.0uf all in parallel near the uC power pins. If the problem persists you'd want to do some power rail filtering and ripple regulation. Consider an LM317 as an option to power the uC alone.
EDIT: Ensure that the motor's power comes directly from your power source and does not share power wiring with the electronics.