equipment:
Yamaha RX-A820
Mitsubishi HC7800
Xfinity DVR XG2V2-P

DVR connected to receiver with HDMI, and HDMI monitor out to projector.

The cable box goes through it's boot up sequence, then powers down and starts over. Never fully starting up.
If I turn off the projector, it will start up and i have sound through the receiver. As soon as i turn on the projector, it reboots and continues rebooting.
If i go directly to the projector with the HDMI and optical to the receiver it works.
the system worked fine for years with a Motorola cable box.
I did a firmware update on the receiver, but still same problem.

There's also a blu ray and Apple TV connected. They both still work.

Any thoughts on why this is happening, and how do i fix it?
the customer is willing to get his old cable box back, but i would still like to figure out a fix for the issue.

I have another customer with the same issue. He also has a Yamaha receiver and Mitsubishi projector. Different models, but same brands.

TIA
Mike