Youtube stuttering, but only certain channels

Hello all,

I have this strange thing with my old Imac (late 2009 model).

Some Youtube videos play fine, and some videos stutter like crazy, and seems like it depends on certain channels.

For example, I can watch videos from Produce like a pro perfectly fine (length +1 hour, in 1080hd) but videos from Rambalac (length less then one hour, also 1080p hd) stutter and pauze like crazy, and are unwatchable.

This only happens to videos at 1080p Hd resolution.

Tried this several times, also after reboots, but it's very clear the stuttering really relates to ceratin channels/ videos.

Ive tried watching them in chrome and safari, were safari runs it a little better. I've updated to high Sierra, and updated everything possible.

Anyone who can help me with this strange issue? I googled it but cant find anything specific to this problem.

Thanx in advance!