If replacing the batt doesn't make the cogging disappear, update the firmware on the Sidewinder to the latest that Castle offers - that software all but eliminated cogging as long as the batts are up to the task.

You might try low power start setting on the esc as well, again, you'll need the CastleLink - you really need the Castlelink anyway so you can set the Sidewinder up via PC and optimize how you want it to run.