I'm a nub so feel free to discard my comments (or answer them), but --

-- have you tried another LBS? they might have a different solution... or tell you if the shifter is defective. (recently had an issue with a defective device; took another LBS to tell me this)

-- what's the reason why you shift a lot from the middle chainring to the big one? I usually do most of my shifting with my right hand. Then again, I am usually on the mostly flat beach boardwalk.