I don't think you are being fussy at all. 300Mbps, is plenty for 4K streaming. 

I’d be checking the output cable and the capability of the screen you are using…can it handle the resolution coming to its input?

insane: No problems for me, and I regularly watch YouTube.

Does YouTube CDN rely on using the correct DNS settings for optimal delivery?

Goosey:

 

I’d be checking the output cable and the capability of the screen you are using…can it handle the resolution coming to its input?

Op has confirmed it works, just not at the times in question.

@Bluntj have you confirmed that it's not a repeat of the mysterious IPv6-related buffering issues from earlier in the year?
/forums.asp?forumId=81&topicId=311317

I'm no longer with 2degrees so can't verify on my connection for you.