RC Car mounted Raspberry Pi runs out of juice
It is uncertain what the exact cause is for malfunctioning, either high frequency noise caused by the motors or a plain power sag due to high current. Here are some pointers:


Does the motor actually have to be run from 5V? Why not run it directly from the battery? 
Consider a separate regulator for the RPi.
Connect a small capacitor (220nF) very close to the motor, to decouple spikes.
Connect a 100nF capacitor in parallel to the 2200uF electrolytic capacitor to suppress high frequency noise.