Is this happening on all S.A videos you're watching or just some tutorials ? Do you guys also experience other drop down when watching other videos from YT or Vimeo ?
Other points to check would be if this is happening while watching the videos during peak time Internet activity : ( other people watching online videos, playing games...etc. ), if you're connecting via Wifi, are you sure you're not experiencing internal Wifi signal drop down ? Have you tried some testings using an Ethernet cable connection ? Cleaning web browsers cache and making sure they are up to date might be something to try as well and finally there's also the computer & especially graphics specs involved when streaming online videos, again ( especially for Windows users ) are the graphics drivers up to date ?
Really hard to tell what could be the issue, I have no such problems here, have a 2 Gig Internet bandwidth from my ISP here ( but it's not fiber, just what they call high ADSL ), connected via Ethernet cable using an Intel Core i7 with an NVIDIA GeForce GTX 980 Ti 6143 MB graphics running both MacOS High-Sierra and Win Pro 10 since I'm using a Hackintosh. Mainly watching S.A tuts on the Mac side of things, and I'm able to watch full screen 1080 resolution videos without experiencing drop downs.
The other point is that if it was an issue on S.A side, there will be a lot more people reporting it then, which isn't the case, so I will definitely try to check what's happening locally, especially if using Wifi.