Sounds like it's just buffering, which will occur if you're connection is: a) not quite fast enough, b) being used by another computer, c) etc.. If you are using the Youtube app, and not the pre-installed web-link that Kobo includes, you ma have HQ mode enabled, which would require more bandwidth, and could cause the buffering. Try turning HQ mode off my pushing the little icon that appears when a video is viewed in landscape. You won't have as nice a feed, but it will probably reduce the likely hood of seeing that little spinning circle
Cheers