Video Quality When Watching On Demand on PC

Hello, I'm hoping I'm posting in the appropriate place. What I want to know is why I can't watch On Demand movies in High Quality on my PC? I have plenty of internet speed. My speed test showed 800-900 mbps. My video card and processer are fast enough too. On High Quality I get Digital Distortion and eratic jumps. I have to set the quality manually to Low quality to watch. If I let it set to Auto it will try to play in High quality and will play distorted. Does anyone know why or have a solution? Or do you know where I can find the answer? Thank you