The Flux uses a sensorless motor. These can stutter, more so if the battery voltage dips under load. NiMH batteries are less capable of dealing with high currents compared to LiPo.
Double check that the battery is fully charged - it will be warm to the touch when it finishes peak charging, NiMHs are prone to false peaks. If the battery is fully charged, then it is likely to be the case that the battery isn't up to the demands you are making of it. A LiPo upgrade would be worthwhile.