I have 6 Roku Ultra players all running the latest firmware. They all plug into a Denon AVR via a (high quality) HDMI cable. The AVR's output to 4k Sony TV's (and 1 Epson projector) via (high quality) HDMI cables. No matter what area of the house we are streaming Netflix/Amazon Prime, the video/audio drops for seconds/minutes/indefinitely. I'm pulling my hair out! It's been doing this for nearly a year.
I have two Ultras, one connected to a Yamaha AVR and the other straight to a small TV. Haven't noticed this with either of them. I do see some complaints on AVS Forums with Roku users and Denon AVRs, but others don't have a problem. I have no idea what sort of settings might be available in the Denon, but that's the first place I would look.
How long are the cables between the Roku's and the Denon and between the Denon and the TV and projector?
Which Denon AVR is this?
Does it have dual HDMI outputs or are you using a splitter?
Do the TV and projector have the same resolution?
My speculations:
1. Your cable runs are too long and/or you're using inappropriate cables for the distance. You probably need cables with Redmere or a signal booster.
2. You're using a splitter that doesn't support different resolutions on it's ports.
3. Your Denon has dual outputs, but they don't support different resolutions.
The more info you can provide the easier it is to troubleshoot!