Thanks for your quick responses! This is very helpful.

In answer to a couple questions/points:

I ignored the "skipping" for probably about 10 minutes. I started riding five years ago and am still learning what's a serious warning sign and what's an annoyance, so that's a lesson learned, I guess: Bike starts shifting by itself, beware.

Bike hasn't been dropped, but it does spend a fair amount of time on a bike rack, driving to and from group rides... I'm very careful with it, but who knows, it's possible it could have been bumped at some point.

When they replaced the cassette, it was with one that was the same size, not larger.

So a couple of you mentioned a stiff link in a chain as a possible cause. How does this happen, and how can I prevent it? I'm good about keeping the chain lubed, but should I be inspecting each link when I apply lube to make sure they all move smoothly? Or are there other ways to prevent stiff links? What do I do if I find one?

Thanks again for your help! Really appreciate it.