Youtube on Firefox/Chromium periodic audio breaks

Mate 18.04 on 3B+. I'm experiencing periodic short audio breaks when playing videos on YouTube with either Firefox or Chromium.

I noticed it was getting out of swap space, so I increased the swap; that fixed the OS from getting stuck but didn't fix the issue of the audio breaks.

I tried installing lxde and login to lxde, that removed the need for swapping at all but didn't solve the issue of the audio breaks.

The CPU usage is not 100% and there is free RAM, does anybody have a clue what may be the cause of those periodic audio breaks which are very annoying, especially if you see a music video?

UPDATE: moving to Raspbian the problem disappeared; I found that h264ify is installed and enabled on Chromium though this may be the difference