If you are using IDL as Tony alludes to, then this is from the manual and may explain why your shadows are jittering:

Animation Considerations: When rendering out an animation while using IDL, make sure that multi-threaded rendering is not being used (set Number of Threads to 1 in the Render tab of the General Preferences dialog). If you leave the setting at 2 or higher, your renders will come out jittery, as the shadows and lighting will not line up from frame to frame.