OK, so some significant progress. That I hope might be of use to someone else. I was using https://www.pingplotter.com/ to chart the pings, it's free for 14 days. I was doing this ping from a macbook...

I'm having an issue of a perfectly regular ping spike when using the Deco M9. The chart below shows a ping to the main Deco. You can see the perfectly regular ping spikes. This is just a local ping...