I hated that problem. I must have changed those damn headlight motor assemblies a million times. There could be one of two problems. First, the plastic gears in the motor assembly could be worn. Second, the catch bar that stops the actuator arm from stopping could have broken off. To check, open the hood and look at the actuator arm that pushes the headlight up. Now take the plastic cover off the manual headlight crank. Now crank the headlight up by hand and watch the metal arm. If the arm just goes around in circles and the stop tab isnt there to stop the arm, then thats what you need to fix. If you crank the headlights up manually by hand and you can feel the gears inside the headlight motor not catching, then the gears are worn out and you can either buy a new assembly, or buy the brass headlight motor gears and rebuild the assemblies yourself. Its not hard to do. Heres the website;
http://www.bfranker.badz28.com/headlightfix/index.htm
I used these gears in mine and they lasted a long time. Actually, I never had to replace them again after replacing the plastic ones with these, I only had to do the stop tab once.