There don't seem to be any techies around, Scotters. I think I can guess what's going on though.
You should have no trouble with a 7mb Broadband. I can only get 2mb here in the Devon countryside :o(
Most likely the problem is with their servers at the other end. That, and possibly huge demend from other downloaders........... especially in the afternoon after the US has woken up.
I think the jerkiness is when they "drop frames" to enable the stream to keep up......... actually skipping frames to keep up to speed. Small screen takes a certain amount of data, but large screen takes massively more, which their servers can't pump out fast enough.
Similar with pixellating. Reducing data to maintain throughput.