make sure the cables travel freely and quickly, especially when the cable is moving in the "slack" movement, or in other words, when the spring of the derailure is moving the cable. also, check to see that all the wheels of the derailure are running parralled to each other. Last gost shifting I has was because the bottom wheel in the derailure was bent just a little out of line. Just before a race! bent it straight by eye and didn't miss a shift the whole race.