Have you adjusted the limit screws? Could be that the mech's pushing the chain slightly too far out when you shift.
Also, do your chainrings show any sign of wear?
Park Tools have got a good page on mech adjustment: http://www.parktool.com/blog/repair-help/front-derailleur-adjustments
Have you adjusted the limit screws? Could be that the mech's pushing the chain slightly too far out when you shift.
Also, do your chainrings show any sign of wear?
Park Tools have got a good page on mech adjustment:
http://www.parktool.com/blog/repair-help/front-derailleur-adjustments