youtube play with short pauses

Video typically buffers when the computer is not able to download data quickly enough to keep up with the video playback.  This can also be the result of CPU contention, where the CPU is too busy to decode the data stream and render it on the screen.

In the first case, this could be a connectivity issue between the computer and the router, the router and your Internet Service Provider, or between your ISP and the server publishing the video.  If you're using wireless, try plugging in with a cable.  If the issue goes away, you know you need to look at your wireless connectivity situation.  If you have something like a cable modem and a router, you might also try plugging directly into the cable modem.  Personally, I seem to be replacing wireless routers every 2-3 years across all the major brands because they get flaky.

Also, YouTube has largely moved to HTML5 video.  You should be able to confirm this by right-clicking on the video.  If it says About Flash Player, it's Flash.  Otherwise, it's HTML5 video and we're not involved.

You can try working through the video troubleshooting guide below.  In particular, check out the part about making sure your video drivers are current.

https://helpx.adobe.com/flash-player/kb/video-playback-issues.html

If you're still stuck, there's probably not a lot you can do beyond looking at newer hardware and/or having a conversation with your ISP about network speeds.