Hi,
I don't know what you have for a hookup with the electrical system, but your problem could well be in the system and not in the motor.
Check in this order.
If you are using the original key lock switch they are known to be a problem area.
Loose connections, check that every thing is tight. This includes the connections in the control box.
Bad battery, try another battery. If your battery is run down and on it's last legs, this could be the problem.
Bad contacts in the relay, if you have a multimeter check the resistance across the contacts, it should indicate zero or very low resistance. If you don't have a multimeter try and connecting the motor directly to the battery, you need some form of switch, a hefty one not a micro switch, to control the power and cut it when not needed.
Check it out, you have nothing to lose.