Hi all. I have a new road bike (~100 miles on it) and when I'm in range of the middle three cogs on the 39-tooth chainring the rear derailer seems to hunt between gears. It seems that the pedaling force causes the derailer to quickly shift in and out of the gear. It only happens on those three gears though and pretty much makes them unusable. The shifting itself seems fine with all other gears. Can anyone shed some light on why this is happening and what I could do to fix it? Thanks much!
It sounds like your shifting is going a little out of adjustment, which is fairly routine for brand-new bikes.
Try turning the cable-tension adjuster on your rear derailleur about one revolution counter-clockwise as viewed from the rear. Or let your shop know that it's doing that, and they should touch it up for you for free, being a brand-new bike. HTH