I'm using the chroma key effect on a photo of spaceship, combined with a pan/crop keyframing, to give the illusion that the spaceship is moving across the screen. The effect is working as I'd hoped, except that the movement of the spaceship is quite jerky.

The timeframe on which the ship zooms across the screen is quite short - about a second or so.

Is there any way to smooth out the movement of the pan/crop, or a way to make the spaceship fly smoother, given the effect I'm trying to create? I've tried adjusting the smoothness of the keyframe interpolation, and haven't had any luck there.

Thanks for any help.