Beware everyone that Google put me back in the HTML5 after I opted out. I've now opted out a second time so we'll see how long it lasts.
Thank you, Lee (leejbarker32). Leaving YouTube's HTML5 Trial has solved my problem as well. It started a few weeks ago and I couldn't figure out what was wrong.
YouTube videos under 720p were no longer playing in Safari. While they played fine in other browsers. If I changed the video resolution to 720p, they would play in Safari but that is not always an option. I can't believe how much time I wasted in trying to resolve this issue.
I completely forgot I was in YouTube's HTML5 Trial. I would never had thought of that.
Now that I left the trial, YouTube videos are playing instantly again.
Thanks again, Lee.