I had the same hair pulling frustration that I'm reading about here. Is it my Apple TV, and if so why don't I have similar issues with movies rented on iTunes? Is it Netflix, and if so why don't I have similar troubles with movies watched on my iPad? Is it my router? Is it Comcast?
I'm sure it could be any of these things but for me it was Comcast. Even after multiple visits from their support techs who couldn't find anything wrong (sometimes it's intermittent) there are 2 things that made all the difference:
1) Continuous Internet connection monitoring with Network Uptime Monitor (http://www.netuptimemonitor.com/) which logged when my connection quality fell. I provided results showing the time and duration of 315 failures in a 24hr period.
2) Writing a polite email to we_can_help@comcast.net providing the results from #1 above. Unlike calling the regular customer service number they leapt into action. I'm thoroughly impressed with their response.
While I think that part of the problem is that Netfilx doesn't buffer enough, when combined with an unreliable Internet connection (even if lightning fast) the issue is really compounded.