Voltage regulator 'hiccups' when gate pin of MOSFET is touched
Steps to diagnose / eliminate the problem:


Add a 10 μF, 6.3 Volts or higher capacitor in parallel with C2 from the 7805 output leg, as close as possible, to ground.
Add a 100 uF, 10 Volts or higher capacitor to the 9 volt input, to act as a buffer / reservoir capacitor. 
Add a 100 Ω resistor on the line from the regulator to the MOSFET gate, and replace the pulldown resistor on the Gate with least 1 kOhms. This will reduce the current spike due to capacitance of the MOSFET gate - I believe this should not be the problem, but it is a simple and useful preventive step nevertheless.


If the above does not resolve the issue:


Use a separate 9 volt supply for the motor, your existing supply is unable to cope with the starting load (stall current >> operating current, usually) of the motor.