Low Youtube quality

Hi, I’m using a laptop (Windows 10) that’s plugged into my router through a chord and my internet speed is over 90 Mbps (Ookla Speedtest). Yet, when i watch videos on Youtube (Google Chrome) and want to watch them in 1080p, it constantly switches automatically down to a lower quality (360p or 480p) as if my internet speed were lower. When I manually switch back to 1080p i ofter works perfectly for
a while. Does anyone have an idea why my pc/browser chooses a lower quality and if there is a way of fixing it?

Thanks in advance 🙂