The video will not play until the completion of the advertisemen

Sometimes totally stopped. I have to click another video and then click back the video I want to watch. If the advertisement is by luck a different one, slow but not totally stopped, I will be able to watch the video. The advertisements inserted in the video are also slow but slightly better than the beginning advertisement.

All the three computers are the same. The internet download speed is between 25 and 30 MB. As I said, watching the video itself never has problem.