You will first want to figure out what server it is talking to receive that video; you can do this with
netstat for instance, or use more advanced artillery like
Once you have done that you can use
pathping on Windows, alternatively
WinMTR) to do a trace route analysis. With this you can inspect at which hop the packets start dropping. And figure out whether it is your computer, your network, an intermediate hop or that YouTube server.
If only YouTube shows this problem, don't bother too much as it most likely does not lie with you computer; you can always try to verify with someone else using your ISP that lives very close near to you (to avoid him from having different hops). If you identify it is an intermediate hop, you can attempt to contact the person responsible for hosting that hop; the hop names often contain the company hosting them. If you only get an IP, you can try to do a reverse IP lookup.