Comment by kroltan

Get the "Enhancer for Youtube" extension, among many adjustments, it does this clicking for you.

I also had this issue, videos would frequently wobble down to like 240p or whatever, on a stable, high speed wired connection.

It's not an internet problem since I never have to buffer when using this forced setting, so it's probably YT trying to save a few bandwidth bucks when they think people aren't looking.