Hello,
i've got the same with a 22418 and 1521/1d with flightpowers 4900 4s,
the only way for me to solve the problem was to put the controller on "low rpm" mode or "race mode"; don't know if the problem comes from a bug of the controller, or if it's just
that the voltage of the battery falls down under high loads (in this case my solution wouldn't be recomanded).
Let us know the results if you try with bigger batteries...