Thank you Carl. Firstly, my hardware and setup:
Modem: BT wireless hub
Roon Core: version 1.7 (build 528) the latest version
Core running on: Apple iMac, retina 5k, 27 inch
Processor: 4.2 GHz quad core Intel i7
Memory: 32GB, 2400 MHz DDR4
Music stored: Meridian Media Core 200
Music output: Meridian 808 with ID41 network card
iMac to router: Wi-Fi
Media Core 200: Meridian speaker-link from network port to BT router
Meridian 808: Meridian speaker-link from BT router to ID41 card
No issues with the wi-fi signal and my download speed is around 23Mbps; upload speed about 3Mbps. Not sure if that is relevant, so just for info.
The Meridian 808 is in a separate room. Not sure of the cable length - speaker-link is a flat cable that runs under a carpet - maybe 15 meters. I have used this set up since July 2015 with no drop out problems. I have subscribed to Roon for just under a year and prior to that I used Meridian Control Mac to run the system. Control Mac is a 32 bit program that no longer runs on 64 bit Mac hardware and that was the reason I changed to Roon.
This is a very recent issue, which started a few days ago. Music (or radio) drops out without warning. The Meridian 808 “disappears” from Roon zone and then reappears usually with 15-30 seconds. The data transmission from the router to the 808 appears constant - by that, I mean there is a solid amber light and a flashing green light on the ID41 network port even when the music has dropped out.
All router connections have static IP addresses. I have logged into the router to check and also use an app called Fing, which shows the IP addresses.
I also have a very recent issue (last couple of days) with my iPhone 11 Pro no longer connecting to the Roon Core. When I click on the app, it shows my iMac as the core; I get an amber button and Connecting, then a red button with Connection Failed. The iPhone software is 13.3.1. I deleted the app and reinstalled it, but still no connection. All other network aspects of the iPhone - internet, email, messaging - work fine. My wife’s iPhone 7 running the same software connects to Roon with no problems, as does my iPad. All very strange!
I have rebooted the iMac, rebooted the iPhone, and disconnected and restarted the Meridian 808. The only items in the chain I haven’t rebooted are the router and the Media Core 200.
Hope this is sufficient background - let me know if I have missed anything.
Thanks for your support.