units disappear

[muddpuddle]

i'll be in the middle of a map/battle
and one or two units will visually disappear.

they will still exist, their banners will still fly, etc,
but there is no figure/army where they should be.

this is making the battles really frustrating because its hard to tell if i've got anything left to move unless i run through the units using the next buttons.

anyone else have this problem?
any thoughts on how i fix it?
i'm running xp w/2gig ram and nvidia 7600.
i even had to turn down the graphics to medium just to get things running smooth. very odd.
this is a great strategy game but graphics issues are really annoying...
any patches for this? i can't seem to find any patches anywhere.
thanks.