Could be a bunch of things, including the chain starting to pop a plate. Check it very carefully in the stand for any thing out of whack. Then go test it in the problem gear, when it does it, stop, no shifting or back pedaling, get off and look.

New chain and used chain ring? It might be an odd big ring suck problem, where the chainring is pulling the chain up, that would give you a lock up feel as well.