Whats Sonos doing when this all happens? Can you completely remove/turnoff Sonos and still have problems.

With Sonos, it is (use to be) possible to inadvertently create a network loop that can flood the network if there were multiple routes between devices - ie via hardwired and via Sonos mesh. The prevent this, some network devices implement a spanning tree protocol (STP) but not all. I had a set of Netgear routers that did not implement this and caused all sorts of weird and wonderful network problems. Not easy to spot this information, STP often buried in protocols supported section of network kit spec. So I changed switches to a different Netgear (I cant remember if the problem was the plastic switches or the 100Mbs switches), and problem gone. I could have removed Sonos from being hardwired too, but for reasons long forgotten, didn't want to. If you have more than one Sonos device hardwired to the network, this could be causing problems.

Any chance of moving all kit to one location to test? 818, MC200, storage, ISP/router/wireless access point, switch, cables etc. Sounds like you might have more than one problem going on so putting everything together then starting to reassemble one at a time will reduce confusion. I'm struggling to see which bits are common with failures other than MC200.

You also mention Sky. We had Sky as ISP and both their white and black hubs were awful for Sooloos. You need to be 100% sure that you keep anything Sooloos over Wifi related out of any Sky supplied hub. So if you are running a Sky hub and wifi, you are possibly into issues. ie Scenarios 2 & 3.

Please also don't get a smart/managed Cisco switch (or any smart/managed switch for that matter unless you know what you are doing). Its like installing a nuclear reactor as a home power supply. Its complete overkill and going to cause more unknowns than it solves. There are reasons why employers ask for Cisco certification when using Cisco kit.

Meridian owner since 1992
Prime & PSU, Focal Elear headphones, Roon (ROCK 8Gi5 NUC), Explorer 1 & 2, F80. 200/203, various Sonos (via Roon).