Try switching motors. If the motor still acts this way on the new location then it's a bad motor.
try another esc on the motor, as Stephen suggested. If it IS the motor and not the esc, then...
1. Undo, and re-connect your bullet connectors
2. Re-solder the bullets onto the wires
3. resolder the wires at the Motor side.
4. If this sill does not work, then the motor is likely faulty.
In my experience a "chugging" motor is usually an indication of a bad connection somewhere.