I have an Apple TV with the same issue. My other devices stream Netflix and Amazon fine (and they should with a 100 mbit connection).
I contacted Netflix's live chat and phone support several times to see if I could get any information. The live chat reps were all useless. Their response to everything was linking to a simple troubleshooting page that tells you to restart everything, or they just flat out blamed the ISP.
When I called phone support, things got interesting. The first rep said there was a nation-wide SuperHD and HD outage, and that only limited areas were up at the moment, but engineers were working on it. She insisted this was true and that all the reps have been getting daily internal notifications about it. Right after the call, I booted up the 360 and had crystal clear SuperHD.
So, I called back, got a different rep, and he never heard of this "outage". When I told him about it being an issue specific to the Apple TV, his initial response was to "restart it", "call your ISP", etc. When I explained to him that all other devices and services were working fine, he did a 180 and said they were "aware of the issue" and were "monitoring the situation". In all honestly, it sounded like a load of BS (just like the talks with previous reps).
I'm beginning to believe the Netflix support staff has little to no information about any issues with the service, and is only trained to say whatever is necessary to end the call or chat.
I might attempt to downgrade to 5.3.