This seems to only be an issue when I've been editing a particular page for a
while.  The problem is that when I have the "custom animation" tab open and I
undo something which involves an object with an animation in it.  The screen
locks up for a bit and the custom animation tab rapidly refreshes for a while. 
After 10-30 seconds, the undo operation completes successfully, and the custom
animation tab displays correctly.  The same undo operation can be performed
almost instantaneously when a different tab (such as master pages) is selected
instead of custom animations.  I had quite a few ( > 20 ) animation effects on
the page when I did this.

Thanks,

Manfred