I had/have a very similar problem as what you describe.
Or maybe not exactly the same, I'm not so sure from your description.
In my case, the video would completely hang. It would maybe play 1 sec or so and then just hang. Moving the mouse sometimes plays a few more frames. It's somewhat strange.
I thought this is a problem with my graphics somehow (also because it noticed the problem since a Nvidia graphics driver update, and I had other graphics problems before, like this).
I thought this is an issue with Chrome but Firefox has the same problem.
Also disabling hardware acceleration in Chrome or doing sth like that did not help.
But then I switched the audio (I attached some Bluetooth headset), and suddenly all the videos play fine again, without any hang. So it was likely the PulseAudio issue I described here.
inxi -SG
. Did you try tosudo ubuntu-drivers install
?