Just because you changed your network settings to class A, it doesn't mean the infrastructure can handle all the traffic, this why some routers/switches cost 10x the price of another router/switch, but well since the miners completely shut down, I would rule out the network thing altogether.

I would like to hear more explanation on the "shut-down / turn off f" part, does the PSU itself shut down and its fan stop spinning or does the PSU shut down its main rail and keep the 12v running so the miner's fan and control board work but the hashboards don't?

Since you mentioned that you changed the control board and PSU, what I would do is move one of the bad miners to another rack, just to ensure that this isn't an electricity-related issue, if it is not, then chances are one or some of these boards have a loose heatsink which loses contact with the chip when the mine gets warm, this is very common with the 17 series and could be with the 19 series too since they gears are new, we don't have much info about them, to troubleshoot that you will need to run the miners with 1 board at the time to figure out which board/boards are causing the problem.