How to deal with voltage sag from a motor starting
Ideally, you are not supposed to have analog(motor) and digital(microcontroller) components on the same power rail as such noise affects the operation of digital circuits. The voltage drop is seen because, momentarily, the motor draws more current than the regulator can supply, and hence this results in a voltage drop. After the motor starts rotating, the current required is lower due to the back EMF of the motor and hence the voltage is stable. 
Consider adding a capacitor across the VCC and GND of the 8V and 12V. You can start with a 1uF capacitor. This will reduce the momentary drop. If it is not enough, you can try a higher capacitance.