Ditto for me viewing US Netflix from Australia. Used to be perfect HD quality on the ATV. Now it either is terrible from the start of a video, or pauses to buffer after a few seconds and switches to a barely watchable resolution.
If I switch to the Xbox360 app (which is connected via ethernet to the same router the ATV is, also by ethernet) the quality is perfect - consistent HD, no stops. Same perfect when viewing via browser (Safar, Chrome and Firefox all tested) on a desktop connected to the same network via ethernet, or via the iPad app over wifi (on the same network).
I can't see how it has anything to do with my ISP or my internal network if Netflix streaming speed and quality is perfect on every device in my house except the ATV?