Denon AVR-S960H receiver Repair

The Denon receiver flashes from normal to black every few seconds, sometimes with a strange "zebra" image flashing to black and sometimes from HDMI2 to HDMI1. Sound flashs on/off with the image This is unstable and occurs 9 times out of 10 for about 30 minutes before stabilizing to normal operation.

Is it a warm up question? After watching TV for several hours in normal operation, the problem reappeared for another 30 minutes after a short automatic upgrade.

All possible tests and resets have been done, and using another similar AV receiver to isolate the problem. Through my Epson 5050UB projector, Videotron 4K cable service, Apple TV and streaming from a MAC computer, all produce a perfectly stable signal. So the Denon AVR-S960H is de culprit.

Do any members know about this? All info and comments welcome, Thank you.