OK - I'm out of possible solutions. I looked at your other thread on this issue. To me, it just looks like you've got a recurrent glitch, not a programming or action problem. There's no reason for the tabbing action to act differently - a video element is just another element as far as the program is concerned. The only 508 issue I ran into building a course using multi-slide videos was that the dual navigation controls (playbar and navigation buttons) could be confusing for a user.
The only way I've overcome stubborn issues that I can't recreate anywhere else has been to delete the offending slide, resave the project, close Captivate, dump the cached versions, reopen Captivate and the course and rebuild the slide from scratch.