Are you running 4.4.3 and having this problem?
And the only thing tried at Apple Store was replicating the problem, which they couldn't. The guy brought out a TV, hooked everything up, attached my ATV2 to Ethernet and I tried streaming some HD TV shows. It all worked fine... but it was working fine at home for me too just before I left. =P
But then last night around 9pm, with my new ATV2 replacement unit, I couldn't even get a 20 minute episode of Family Guy to START streaming... a nice big fat 1 hour 52 minute wait was in the way. The ethernet connection my ATV2 was getting was 18-20mbps...
I'm pulling my hair out trying to figure out what's wrong. The only conclusion I can think of is that there's something wrong with the iTunes Store server itself... >=(