Mob inside mob spawner "stutters" if no player is in spawn range or spawner is outside worldborder

Once the spawner's model starts spinning, the model will never stop moving again. It will twitch rapidly back and forth instead.

This is most visible on Ghast spawners (and might not be noticable on spawners for any other mobs).

This is visible in many youtube videos of CTM map playthroughs. See http://www.youtube.com/watch?v=pqsrB-4xC8s#t=23m52 for an example.