I'm going to jump on the "front derailleur isn't adjusted right" bandwagon.

My recommendation is to start completely over. I could teach a chimpanzee to adjust a rear derailleur but fronts are much trickier. I think that the heart of the issue is that each front derailleur tweak affects subsequent adjustments so, once one adjustment isn't perfect, there is a tendency for making subsequent mis-adjustments.

I'd start by completely disconnecting the shift cable. I recommend the step-by-step instructions on the Park Tool site. Pay particular attention to the position of the derailleur on the seat tube, the very first adjustment of them all, because that's where I've found the problem usually lies.

Good luck.