If the Archer C5400 is in deed a switch, why is the following scenario happening?

Desktop <Cat6> Switch <Cat6> C5400 port 4 <Cat6> Cable Modem - All Hard wired

File Server <Cat6> C5400 Port 1 <WiFi, about 6 feet from router> Media Player

If I do a serious pull from the Internet on my Desktop, the connection between the File Server and Media Player goes to complete crap. Anything on wifi, doesn't matter if it is using the 2.4GHz radio or either of the 5GHz radios goes to crap. As soon as I bandwidth limit the desktop connection (my Internet is 300mbit), the Media Player returns to normal. If the C5400 LAN ports are indeed switched, this should not be happening at all as ONLY the C5400 is a common transmit path through the whole network, and the two traffic paths are divergent and never cross.

Is there any reason that this should be happening on this high end consumer device, or do I need to actually put a real switch in front of it to isolate my internal traffic from my Internet based? My previous routers all handled this properly like they should, but the several hundred dollar device just doesn't seem to actually handle it correctly.

I am on the 20170630 firmware, and will be upgrading that once the current movie is over. None of the fixes shown in the Bug Fixes list pertain to this issue, but I do have the problem of the router forgetting its configuration when power cycled (like when the power company just changed my meter and I lost all config and couldn't find my backup).