This then excludes any hardware, operating system or browser issues. In the event of limitation or hardware or software failure of your computer, the ads would not be broadcast in HD either. I have noticed specifically with Amazon that the connection seems a little uncertain and that my quality is from HD to 240p-esque on a DL disk at 320 Mbps. I do not think my ISP has an Amazon Prime cache server like it does with Netflix, so that may have something to do with it.
Whatever it is, it seems like it's still due to your connection. Is this machine a laptop or a desktop? Do you use a WiFi or Ethernet connection? If it is Wi-Fi, the WiFi driver may work differently than the one used on Windows?