I have had the same issue with Netflix from the beginning. Here is a theory that may answer some of the questions on this discussion. I have never had an issue with locally streamed content from within my LAN. I have also never had an issues with content purchased directly through Apple. I have had this issue consistently with Netflix streamed content. What I have noticed is that if I purchase an Apple movie, I can pause it, walk away for a while (15 minutes or so). When I return, I will see the movie has downloaded all or most of the movie.
When I start a Netflix movie, however, no matter how long I pause, the movie doesn't buffer. I think this is a limitation that Apple has built into the device. If Apple allowed full buffering for all content, this wouldn't be an issue.
Anyways, this is my two cents worth.