Okay, so I was just able to 1. successfully download a purchased movie without the download failing, and 2. watch said movie without having any of the issues above - no stuttering, crashing, constant pausing, etc. I also haven't gotten the port 443 error message all day either (knock on wood....)
I also just started to play a movie I haven't downloaded locally, and none of the issues occurred with that, either. Fingers crossed.