Trembling units animation bug

[MarkusAntonius]

i've tried to find something about it. but this bug seems to be unknown. here is the bug i'm talking about
http://www.youtube.com/watch?feature...&v=LfBJTMWux6Y the units, ships, campaign units, 3d portraits and the advisors tremble without stop. actually their animations change so quick that make them to tremble from normal to dead and mounted animations and to rotate and levitate. all of them happening in seconds. someone knows what causes this and how to fix it ?. happens on battles and in the campaign map units and ships. but not on the historical battles at least one or two times with the 3d portrait but in minutes very different from the rest of the game.