I am in the same boat. About two months ago, Netflix started performing horribly on ATV (both on my 2 and 3).
Strangely, it streamed fine on the iPads and on the MacBooks but was just as bad on the iMac as on ATV.
I tried moving the router around and that didn't do anything.
I figured out I could stream to an iPad and then send to ATV via Airplay and get nice HD quality. But recently that has stopped working reliably. As in, it would just refuse to send the content to the ATV. Even now, trying to play straight from the ATV Netflix app prompts the "there was an error loading this content" message.
I just tested the app on my Bluray player and it works fine. So it's definitely not JUST my internet connection. Yet at the same time, my cable connection in general has been acting goofy the past few weeks, so I wonder if there is some issue with the signal that could be causing a problem on the Netflix app (could it be more sensitive to signal idiosyncrasies there than other versions?) on ATV and not necessarily elsewhere.
It's all been monumentally frustrating. Everything used to work fine. Now I get to points where I'd sooner cancel all of these services and not have to deal with the hassles.