Nvidia Control Panel. If it is set to adaptive power mode it may by pulling voltage from the cards when load drops, resulting in stutters in changing game environments.
Open the nvidia control panel, go to "manage 3d settings" and select the program settings tab. Drop down the list to find your game, and then in the box below find "power management mode" and change that one. Adaptive allows it to downclock when it is under lower usage. Set it to Prefer Maximum.