At peak times it isn't out of the question that the video service itself is a bit under the weather. I've had some troubles with Amazon at peak time, and it's always been with fresh content, such as yesterday's television show that just became available on Amazon four hours ago. It wasn't obviously my network, as switching over to Netflix worked just fine at full quality. (That's not a proof it's not still somehow network related, but it strongly suggests it wasn't my ISP.)