Might be the freewheel rachet slipping, although I haven't heard of it happening like this before, usually you just go to full coasting. If the cassette wobbles, it could be lifting the chain up and then dropping it down, kind of like being between gears. Cassette wobble sounds more like a loose rear wheel bearing to me. That will lead to other problems down the road. All in all, it depends on how much longer you want to ride this bike. If you plan to ride it into the ground and get a new one, probably doesn't matter that much. But if you want to keep it a long time you should look at addressing some of these problems, like chain skip.