Check the chain. A tight link? You said you changed it but if you joined it with a riveter (vs a quick link) you may have left the riveted links binding a little which would show up first on the cog that requires the most link "bend". I drive the rivet through to its correct protrusion on the far side of the link which leaves a touch more rivet on the near side, then place the link over the middle position on the riveter where the far plate is not supported and press the rivet to equal to its neighbors on the near side, putting the play back into the links. Shop mechanics often simply flex the chain side to side to get that play back.