Issue with playing HD Videos

Hi Adobe community,

I just bought a new Laptop (Lenovo S440), Windows 8.1 64bit, installed the Firefox web browser (35.0.1), downloaded the Adobe shockwave flash (16.0.0.305). Now I am trying to watch a Youtube video (VICE on HBO Season 2: Crude Awakening and The Enemy of My Enemy (Episode 9) - YouTube) on HD 1080p and I am experiencing constant pausing and buffering (around every 30 seconds). My internet connection is above 20mbit/s, so that should not be a problem. I measured it with the Ookla online internet speed test. When watching the exact same video on my brothers computer on 1080p it works smoothly, he is connected to the same modem as I am. I do not understand why I am experiencing this issues, my Laptop has i7 processor and is new, he is using an older PC with the i3. I already tried to uninstall the Flash and Mozilla Firefox and reinstall it, but that did not help (But I could not completely remove it in the way they describe it here https://helpx.adobe.com/flash-player/kb/uninstall-flash-player-windows.html, because I could not delete the files in C:\Windows\system32\Macromed\Flash, it said I needed permission from the TrustedInstaller to delete the files, I included a screenshot of that). I also compared watching a Twitch stream (Twitch) on the highest quality "Source" my laptop as well as on his computer, same story, it works smoothly for him but it keeps pausing and buffering on mine every minute or so. I really do not understand why I have this issue, as my laptop should actually be much faster than his. Another big problem is that online movie streams do not work at all. For example https://archive.org/details/97154_hm_eastern_homes keeps buffering for 15-20 minutes before it starts, on html5 as well as on flash (you can decide which one to use under the video). What am I doing wrong? It is really frustrating because watching online media is a big part of what I bought this new expensive laptop for.

Regards and thanks for any help,

Adrian