Instabilities with GS1200-8 managed

Thanks for asking, Adam.
Yes, switching off both Flow Control and port based QoS made it rock stable. I could try to switch on Flow Control since by itself it did not seem to make the difference, but I don't dare touching it since it works so nicely now. Since the gigabit switch has enough headroom for the 100mbit internet connection I would probably not see much difference fine tuning those two features anyway.

Like outlined above, the flanky speed issue with the router was just one. Also problematic were the seemingly sporadically dead ports on the other devices. But anyway, solved with these settings.