My guess is, UPC just has the occasional overbooked peering link problem. A while ago I was frequently watching youtube videos, and found the quality to drop considerably in the early evening hours, when probably many other customers also started to watch youtube. Enabling a VPN to the UK, sometimes even to the US, brought back the quality. So, in my interpretation, my home connection was NOT the culprit, but the backbone interconnection of UPC with the peers I was watching content from.
I haven't had such frequent issues recently, but then I also mostly switched to watching twitch instead of youtube, and that will go over different peering links.
Cheers, Markus