My Trek 3900 RD is acting up a little. When i shift, nothing happens unless I shift TWO gears, not just one. Then it shifts as smooth as butter. This seems to be the case in either direction, up or down.
The RD is a Shimano Acera, which is nothing special, I realize, but it seems to get the job done. Anyway, my question is: what's likely causing this, and is it a simple adjustment even a knucklehead like me can do, or does it probably require a quick trip to the LBS?
BTW, they replaced my RD at 900 miles or so under warranty. This one has about 1,100 miles on it. The old one was doing the same thing only much more annoyingly (and would sometimes shift unexpectedly maybe 300 yards after I shifted -- SURPRISE!). Back then, I didn't ask what the adjustment was; I just took it to Trek.
So...what's going on, and can i fix it?