Probably silly to answer my own question, but I think I've discovered the culprit.
The PPT presentation contained a number of animation sequences that did not convert properly into Captivate 7 during import. (I had removed several of them, but left a few assuming CP7 would know how to handle it.) Well... As soon as I edited the source PPT slides and removed the animations, everything started working as expected. 
So - the outstanding question may really be whether or not CP7 can handle animations imported from PowerPoint 2010.