I have now uninstalled Avast and replaced it with Avira... and I don't have any of the problems mentioned above. More importantly, Youtube videos don't stall or stutter the downloading (the video plays fine, of course - I mean "stuttering" of the download progress).
I really hope Avast fixes this, because I recommend software for many friends and family, and they all use Avast because of me. Some have purchased it as well. I would hate to give up Avast, as it has been so good for me in the past. Please fix this issue... and let us know if it's being worked on or if you need any more info, please!