Definitely look at the chain where it was joined. My wife's bike started exhibiting exactly the same thing you are seeing. It would skip a gear here and there. After a a few more rides it would barely stay in any gear for any amount of time. She was not happy, it was taken into the shop twice. She finally made the shop owner/mechanic go for a ride with her. He immediately recognized the stiff link and repaired it quickly. She hasn't had an issue since. Oh, and that chain didn't start doing this until she had about 700 miles on it (was new this past spring).