I don't know which forum is appropriate for this, but this one seems more appropriate since this problem only occurs with one browser on one computer.
For several months, I've had problems with YouTube videos randomly jumping forward by what appears to also be a random amount (2-15 seconds). Recently, it has started happening more often, and videos will sometimes stop buffering completely (they stop playing with the spinning loading indicator, but no network activity at all). I've even had videos refuse to play at all (once again, the buffering indicator spinning with no network activity). Based on the timing of when it started, I thought it was related to the anti-ad-blocker stuff (such as the 5-second delay which ended up also affecting Firefox without an ad blocker). However, I've since discovered that there are no problems with YouTube playback on another computer with Firefox and exactly the same extensions. I've noticed that clearing the Firefox cache helps for about two days, and then the problem comes back.
When this started, there were no video playback problems in Edge, but I haven't tried Edge in a while. I tried a live Linux flash drive and had lots of buffering, so I should try Edge again.
Has anyone else seen this problem? I'll add some system information later.