Question Pc has bad video quality only on videos

Typically videos will go low resolution when bandwidth requirements are not met.

I would run a internet speed check on your PC https://www.speedtest.net/. Be sure you do this either hardwired via Ethernet or as close to your wireless router as possible. If you are in fact on wifi then the quality drops could be a poor signal either caused by distance or by too many devices on your wireless network or on other wireless networks like in an apartment.
 

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