It also sounds like the locknut is pressing onto the top end of the steerer tube as you are tightening it. You need to put sufficient spacers between the threaded race and the locknut so that tightening it up doesn't press on the end of the steerer. I also find a little Loctite on the locknut thread can prevent self-unscrewing.
It also sounds like the locknut is pressing onto the top end of the steerer tube as you are tightening it. You need to put sufficient spacers between the threaded race and the locknut so that tightening it up doesn't press on the end of the steerer. I also find a little Loctite on the locknut thread can prevent self-unscrewing.