It's imprtant to know if the chain is staying entirely on the gear when jumping or is instead starting to shift to another gear. If the former and the bike (not the chain) has a lot of miles on it, and you use those middle gears a lot then they have worn enough to skip (the wear is not always easy to see). It would seem unusual for it to show up suddenly, though, especially after months of using that chain.
If the latter, go through the full process of setting up the rear derailleur, including making sure cables are running smooth and clear. Any kind of misalignment of the derailleur or hanger can also cause "jumping" in some gears and not others.