what I was told by the pro mechanic is that you should only have about 1 mm of clearance between the big ring and the bottom of the front deriailler. You might play with that setting, it's free. All you have to do is loosen the screw that holds the derailler to the frame, and most likely pound on it a bit to lower it. You'll want a real accurate measuring device (we used a caliper) to check the clearances. Changing that seemed to help my front shifting woes quite a bit.

Irulan