After a while I have found the problem... SATA power for the risers... is not a good combo.. So after changing those the rig was stable..
Now with replacing some of the cards for newer ones we have new challenges... ;-) Thanks for the support.. really appreciated.
Sata raisers are better than molex raisers (sata power connectors usually have much less issues with bad connectivity than molex) and are good for pcibus power hungry polaris refs
Okay, I think our problem at first was that I connected to many risers to 1 SATA cable, but I also read online that SATA was kind of restricted to something like 9A and Molex near 22A. Therefore I believed it would be beter to have the risers connected to a molex cable and also connect not more than 2 risers per cable. Since I have two bad connectors now on my PSU I can not connect every riser to its own molex cable, I had to fit 2 on one cable. So far this seems to hold. (not overheat)
Soon I will have all the risers connected via PCI-e....
I came along another "challenge"..... After I saw a high number of rejected shares for only 1 card I altered (lowered) the VMR and wanted to restart the Phoenixminer, but when I did this it "noticed" a third (ghost) card (RX5700XT while we only have 2 actual RX5700XT in the rig) on position 2 in the list of cards and the number of usuable mining cards was also added with 1. ..... It does not show in the windows devicemanager list..
Any ideas what could have caused this?