That happens when your headlight motor is going out, mine did that. I got the brass gear from ebay, pulled teh motor out and replaced the gear and it fixed it. The motors almost never break but the plastic gear inside of them strip. Try taking the bar off of the motor and manually turning the shaft 180*with the adjustmentthen put the bar back on and adjust it back to the position your headlight was in before. If it works it will only work for a little while but it will let you know that is your problem.Theres also a write-up on it somewhere.
1968 Firebird 400- LS1/T56 swap in progress