Look at the gradient carefully. The transition of black to green is changing in length with each new frame. I suggest locking the gradient and placing it as a layer under a mask, which in this case could be the cheetah. This would work as long as you're using a program such as photoshop. This might seem like a stupid little suggestion, but really its making the green standout quickly on the final frame. Doing so, gives it a choppy look.