Red Artifacts left on screen when moving with actionscript?

I’ve got several small moveiclips moving around the screen to different locations using the tween class, and for some reason it leaves little red artifacts in their trail on the screen that won’t go away. any way to refresh the screen or something?

Is the only way around this to cache the movieclips as bitmaps? that seems to fix it, but i don’t want to do that…