Tested webcat, sailfish-browser & LLs Video Player. All suffer from the same issue.
The interesting thing. Now at the day time I have a very slightly better connection (cat /proc/net/wireless shows link 58-60) and I can watch videos more fluently. (sometimes when I move I still get some rebuffering which keeps stopping the video playback).
I don't have that problems when getting nearer to the router.
So I guess its simply out of range though thats then the only device I found so far with such poor reception.
I can literally see the router here in the other room and browsing interstingly works fine. But as soon as I try to load some videos. Maybe its the default cache size in gstreamer that causes this issue of videos not being played back fluently.
Though also the sailfish-browser has its problem and that should not use gstreamer as far as I know.