How do I stop Youtube dropping the video quality?

[Quartz]

Youtube seems to like dropping the video quality from 1080p to 360p when I have 80 Mb/s bandwidth. Manually changing the quality setting back fixes the immediate problem but I'd like to stop it happening in the first place. My browser is Chrome. 64 bit, Windows 10 and Chrome are fully updated. Extensions are Lastpass, Privacy Badger, Privacy pass, Google Arts & Culture, Google docs offline, and uBlock Origin.

It's been happening for a month or two now. There doesn't seem to be any rhyme or reason to when it happens and it doesn't always happen.

Any suggestions?

 

[Quartz]

Is this when the video starts? They do this on purpose at the moment save on bandwidth - especially on mobiles, where the difference isn’t so obvious

Good question. I should have clarified. The videos generally start at 1080p then drop down after some random time.

And I'm on wired LAN, not wifi.