I know it’s late in the month to post a reply for this question but, I have come across this same situation several times. Bob is right about making sure everything is very clean, especially the hairspring. If there is any residue left on it it’s going to do just the kinds of weird things your describing. I’m not saying this is the answer but I feel the hairspring is sometimes overlooked. Check it out with a good strong eye loupe. Hope this helps.