I had a similar problem with a rear derailleur and it turned out the derailleur was actually a little bit bent. I also made the housing piece that goes right there too short. They just bent it back for me, and i switched to longer housing, and that did it.

I know its your front derailleur, but it might be some combination of unforeseen problems like that, I'd take it in. I mean I hate taking things in but sometimes you have to admit defeat and learn from it