I've had the same issue with my 105 front derailleur. Mine gets full of grit from the road and becomes difficult to move from small ring to large ring.

Yesterday, I cleaned mine with a citrus based cleaner and a little methanol. After it was cleaned, it worked beautifully. I'm going to try running it with little or no lube and see what happens. I believe the lube causes grit to accumulate in the mechanism.