This issue possible vs others software, so better have it as option
PS You write about not stable exchange. I think it come from old 'data' command, in some rare cases possible receive not calibrated point.
In all cases better use
'scan start stop points 7'
console command it supported by all fw (i add it to eddy v0.7)
PSS also you can select less bandwidth on device and get more better noise floor on measure (in < 100MHz range noise floor for S21 can be less -100dB)
PSSS i made release v1.0.63 for H/H4 https://github.com/DiSlord/NanoVNA-D/releases/tag/v1.0.63
This threads pretty massive and may take some time for you to track it down if you decide to dig into it. You may want to post a question to member Radiolistener. The problem I ran into was random errors in the data. I believe the problem was how they were doing the sweep. From what I remember they had a fixed delay, then changed the delay based on frequency to try and speed it up. I think Radiolistener changed over to using some of the hardware status bits to determine when to move to the next point. After they made this change, the problem was solved. When I decided to release the software, I tried some of the latest firmware but had the same problems I saw from over a year ago. So I am back to using very old firmware that I know will give me clean results.
If you have been reading my more recent posts, you will see me collecting data down to 10KHz. Another change Radiolistener had made that the new firmware never adopted.
Back then I wrote a regression test that I can run against the firmware. So many versions were popping up, it was like trying to follow Dave's 121GW meter. That regression test requires several hours to run but does a decent job weeding out basic problems.
I haven't ran into to many problems with the actual serial data. Some experiments I have ran for hours at a time. The only problem I can think of is with the V2Plus (not the 4). This VNA will hang and require a power cycle to recover. I tried several tests to narrow down the problem but in the end, it was tossed into its box and I have not looked at it since. Too bad really as it seemed like a decent product otherwise.