This only occurs over rough surfaces? If so, sounds like cassette play. If you have a shimano wheelset, utilizing the proper spacer, your cassette should have ZERO play. Please let me know exactly when you hear the sound and when it skips.

In the past I had purchased a new chain for my wife's bike. I found out quickly that we also needed to swap out the cassette because it would "skip" every so often. But this happened over rough and smooth surfaces alike. And you have a new chain and cassette.

Also, I had bad cassette play once. It made a plinking sound that was pretty annoying. However, it never skipped.

There are a few possibilities here but they all lie within the chain/cassette/freehub area.