No, that's not normal. Unless there have been drastic changes in weather conditions you're settings shouldn't be off so much that you can't get past half throttle. Most of the time you should only have to tweak your tuning for the exact conditions of the moment.

I agree that it's probably vibrating out. I don't really know of a solution either.. I'm not sure that you'd want to threadlock it, since you're going to have to adjust it.