When your Youtube videos freeze but audio keeps playing it's usually fixed by disabling hardware acceleration in your browser.
Sometimes you might also need to update your browser version or update your video drivers for your computer and sometimes it could be due to the browser using too much Ram and other resources.
Open your task manager and see if the browser or something else is using too many resources and check to see if your CPU usage is too high.
Your graphics card could also need updated with new drivers or the graphics card could be causing issues as well.
Sometimes just restarting the browser helps clear the problem and also clear your browsers cache and other files where the videos get stored because that too could be causing an issue.
It sometimes happens to me and if the updating drivers don't help I just have to close out the browser and restart it and it fixes the issue.
Sometimes even the browser itself will freeze up and the audio keeps playing and I have to close out the browser through task manager.