I buy plenty of "bandwidth" from TWC but I do not get the streaming quality I expect from youtube or netflix. It always starts out HD enough, but it always fall

I buy plenty of "bandwidth" from TWC but I do not get the streaming quality I expect from youtube or netflix. It always starts out HD enough, but it always falls back to a much lower quality after a few minutes. SpeedTest results suggest this should not happen, but I know my way around the cable company fuckery enough to see that a less-tolled route would probably be superior, even at half the max bandwidth.