My problem with videos seems to have disappeared. I did not update any drivers (Device Manager says they are current anyway) or make any other changes. I opened and closed PTEAV multiple times, rebooted the computer several times, loaded a few older slideshows which had no videos in them and previewed them with no issues. Finally reopened PTEAV, loaded the slideshow which was showing choppy videos, previewed it, etc. No problems whatsoever. Closed PTEAV and also had no problems with any existing videos on my computer.
So it seems like there was something running in the background that may have been causing the issue. I have no idea what as I did search through Task Manager and Resource Monitor at the time but found nothing. Hopefully, the problem is gone for good. If it comes back, I'll see if there are updated drivers offered at that time. For now, my Dell Support Assist says all my drivers are up to date as well. So I am leaning toward some sort of conflict with another program or process rather than drivers. Or possibly a gremlin snuck into the computer while I wasn't looking.
Thanks for the prompt responses from both Jill and Igor.