Are you on a 4mb line? I use my Xbox to stream netflix, and first subscribed when I was on a 2mb line. I was stoked with the way it worked at 2mb, and netflix quality was the only reason I upgraded to a 4mb line. I was pretty underwhelmed with the performance when I upgraded, for double the speed it looked virtually exactly the same.
When I do speedtests I dont use myBB's tester, but
www.speedtest.net . Then select a Californian server.
When I do an EU speedtest to France for example, I get a stable 3.4Mb, its the same for local speedtests but obviously the ping are way better. California, 1.8MB, 340ms ping. I'll post some pings later when I'm back home from work and we can compare. Also I'm using Unotelly for the DNS vibes.
I have heard that other devices can stream better than my Xbox though...especially Apple TV, WDTV and that kind of thing. But I haven't tried those. I'll try on my PC tonight and see if thats better. Maybe its just my Xbox?