HD Video Playback Quality Issue

[Poseidon]

I've noticed recently that when streaming HD videos, the picture quality degrades (pixelated, blurry, low quality) for several seconds and then it jumps back up to HD (720 or 1080). This happens every 3-4 minutes when watching an HD video.

I just noticed this happening after changing my router from N66U to the AC66U.

Something to look for and change in the Asus router's SETTINGS that could fix this issue?

Thanks.
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Win 7 x64
Charter HSI 30/4
Motorola SB6121 modem
Asus RT-AC66U (latest official firmware)
Wired connection from router to PC