But: why is there a Media Network problem with the AVR on?
Perhaps because your Marantz is connected to the network (either wired or Wifi) and is acting as a DLNA device, and is apparently doing something that really irritates MC. Without using a protocol analyzer to examine the traffic between the two devices, I couldn't say what. It's an unusual symptomology, but I suggest this because of your claim about Media Server.
If you disconnect the HDMI link between the PC and the Marantz, and the problem persists when Media Server is running and still disappears when Media Server is not running, then the above is proven. On the other hand, if the problem totally disappears with the HDMI cable disconnected, it's not a DLNA/Network issue, it's a display related issue.
If it's proven to be DLNA related, then you could address it as follows.
Since you are directly connected to the Marantz via HDMI, you probably don't need DLNA on the Marantz.
You could:
1. Disconnect the Marantz from the network (pull the ethernet cable or disable its wifi) or
2. Disable DLNA on the Marantz or
3. Block the IP address of the Marantz in the Windows firewall
Any of these will prevent MC from seeing whatever the Marantz is doing that offends it so.
I've not seen people complain of this problem before. Could be an IP address conflict, but I doubt that very much. As I said, it sounds unusual.
If you want to put the problem behind you without spending a bunch of time investigating, do one of the above. #3 Will allow the Marantz to do DLNA with other devices, just not your PC running MC.
Regarding 15-20% CPU usage, that is not particularly illuminating without identifying the process causing it. You could take a screenshot of the process manager when this is happening, so we can see which process is using all those cycles.