Originally Posted by sam83

At about 3K miles my 6600 (10-spd) quit shifting down the cassette (little lever) reliably. First it was just the last gear, then seemed to get progressively worse. Ended up replacing it.

Make sure you don't have a rear der that got banged out of shape and you may want to take the cable loose and re-check the function by hand. Hope you have better luck than me.