Ajax functionality working sporadically

Hello Paul! Thanks for your great work on this project. It will be a lifesaver if I can get it working consistently with my products. My issue is too confusing to write out here, but I've posted a four minute screencast of what's happening. If you would be so kind so as to take a look, it would be much appreciated. May be something simple that you can see right away. (Forgive the first couple seconds of the audio being cut off...)

https://www.dropbox.com/s/lq29fpkrzkhtdu1/visionart-cart-error.mov

Thanks! R