I had this issue a few months ago, and I'm having it again. If I have adguard turned on Twitch keeps showing me a "Commercial break in progress" video for a bit, goes back to the stream, buffers like crazy etc.
It works just fine when I turn ad guard off, but obviously you get the ads.
Not surprising really, Twitch has been putting A LOT of effort into fighting ad blocking.
BTW I also have Adguard Extra extension installed.