Help with intermittent protect mode (1 Viewer)

I replaced a squeezebox on my Onkyo TX-8511 system with a Denon DNP-720ae.Suddenly, what was a stable setup for 10 years, started having a problem.The Onkyo goes into protect mode for a second or so at high volume levels, in and out.The Onkyo goes into Sylvania SLX-8 which pipes to a number of other zones in the house.I tried bypassing the home system with some smaller 8 Ohm speakers and did not run into the same problem, but the volume on the Onkyo did not need to be turned up nearly as high for those speakers than it did for the house setup.

Nothing has changed on the home system - using same volume level as I have for quite some time.

SLX-8 feeds to in-wall volume dials. Each zone trips the protect mode individually, and in combination.I isolated different output devices through the main setup and still having the same problem. For example, just hooked up my CD player to the system and it enters protect again, but then not with the bookshelf speakers.Checked for shorts, impedance, other connection problems with cables, etc.Unit is not getting hot.
I'm thinking something might be wrong with the amp...

Have you tried bypasssing the SLX-8 and does the Onkyo do the same thing without the switch?

PS - welcome to the forum