Like many other small churches now, we are new to live streaming. I have built an NDI system and we are up and running. However, we have an issue with a black screen flickering in and out. Sometimes it will be a black screen for a second or two, sometimes it will flicker for a few seconds. When there is audio, there will be significant delay just before it starts flickering, then will catch up when it comes out. It is visible in the live window in vMix, and also any output, whether it's a stream or a recording. It seems to occur mostly when lyrics are coming from ProPresenter, and maybe only then. I haven't been able to confirm this. Also, it's not just when streaming or recording. It happens with both vMix and ProPresenter open on their respective system.

Can anyone help? I have a video clip if that will help. It was taken yesterday while I was setting up Sunday's playlist in ProPresenter. Our configuration is below.

Infrastructure:
We have built a gigabit network over the past few years, retrofitting it into an old building. The switches, etc. are all gigabit. Cabling is a mix of Cat5e, Cat6, and Cat6a. Everything on the streaming setup should be Cat6a, except for patch cables.

Camera:
PTZOptics PT20X-NDI set at 1080p/30fps.

Presentation laptop:
15" Retina MacBook (2014 model) running macOS Catalina 10.15.4
2.4GHz Quad-core Intel i7
8g 1600Mhz DDR3 memory
NVIDIA GeForce GT 650m 1gb
Intel HD Graphics 4000 1536mb
ProPresenter 7.1.1
We're at the point of needing a new presentation computer, but are trying to wait if we can.

Streaming PC:
I built this pc, which is dedicated for streaming. The only apps on it are vMix and Studio Monitor. We don't usually have both running at the same time.
Motherboard: MSI X570-A Pro ATX AM4
CPU: AMD Ryzen 7 3700x 3.6GHz 8-core
CPU Cooler: Noctua NH-U12S 55 CFM
Memory: Corsair Vengeance LPX 32 GB DDR4-3200 CL16
Storage 1: Intel 66p Series 1.02 TB M.x-2280 NVME SSD
Storage 2: Seagate Barracuda Pro 10 TB 3.5" 7200RPM HDD
Video Card: NVIDIA GeForce RTX 2070 Super 8gb
Power Supply: Thermaltake 650W

vMix setup:
v23.0.0.58 x64
4 Inputs:
Camera
Audio from the soundboard via USB (to the presentation PC)
Lower thirds lyrics from ProPresenter
Presentation (full slide) from ProPresenter
The camera input is setup with the lower thirds input as multi-view. Maybe this should be an overlay? The multi-view setup is what I was shown. As I mentioned, I'm new to this and still learning.

Flickering and periodic black on NDI is most likely a network issue. So I would check on the network first hand. Try to validate that all devices auto negotiate to 1000 mbps. Sometimes the negotiation just comes out with 100 mbps which would not work. This is easier with managed switches. If auto negotiation is just 100 try to set manually to 1G. Go with quality switches. A switch for 50$ is nothing you want in a production network.

Try a different switch.

Check the network topology. If there is a cascaded network with multiple switches/router/firewalls/etc it is necessary to fully understand the network and check the traffic on all junctions.

You can also try multicast instead of unicast. Make sure the network devices support IGMP snooping to support this option.

I don't know about the NDI cameras, just remember there was a thread about PTZoptics NDI cameras with a problem that sounds similar to yours:

https://forums.vmix.com/...NDI-Black-Frames-on-vMix