hookedonpole, if I remember correctly you are in California. My suspicion is that you are simply having bandwidth issues as California’s internet is overtaxed between failed nodes and carrying communications for the fires that are going on in the North Bay area. The intertubes in that area are struggling to keep up. You will notice less impact on sites like Youtube and Netflix because a multi-billion dollar budgets and infrastructure gives them more ability to scale around issues like this and more buying power to gain priority on limited access connections.
If that is not the case there are several reasons this can happen.
1) Slow internet can prevent your browser from caching the video in the time that it is allowed to do so.
2) A broken browser cache can cause this
3) A malware infection on your computer can sometimes cause this as well
Potential resolutions:
1) You can test your internet speed at http://www.fast.com
2) Restart your browser
3) Clear your browser cache and restart your computer
4) Use a package like Malwarebytes or Trend micro antivirus to test and clean your machine