I know this is an old thread but I'll add to it anyway...
Have you denied cookies for YouTube?
If you are cautious and want to limit your online footprint, you may have elected to deny cookies with YT.
However this also has an adverse effect. Included within the cookie is how often an ad is displayed; 1 ad for every 3 vids that you watch or 1 ad every 5 vids etc - since the cookie is denied, it cant tell that an ad was played recently and so does it on every video that you watch.
Even if this answer doesnt help the original poster, I hope it helps someone