I have a 2008 Madone 5.2 that I bought used. It comes with full Ultegra gears. A couple of weeks ago I tuned it up for the first time using the Shimano instruction manual. Everything worked fine on the first couple of short rides but now I notice when I downshift it takes a second to actually downshift in the middle gears. When this happens it seems to make a noise as if the gear is skipping. Then all of the sudden it will just pop into gear. Shifting up the cassette works just fine with no problems. This seems to only happen in the middle of the cassette (gear 5, 4). Does anyone know what this might be and how I fix it?