I have a very similar problem. I had (2) seven card rigs running fairly stable for 3 or 4 months. Monday morning 5/21 I got notification from ethermine that one of my rigs was off line, looking at the stats the other was only working at half capacity. (2 separate machines both took a significant hit within a 1/2 hour of each other. Looking at both machines checking for any leaking capacitors, checking connections I rebooted, both machines once through the bios, windows would only go to a black screen.
My machines are both Asus Z270a Prime mobo, both have 8 gigs of ram, both have plenty of power, and I'm using MSI RX 580 armor cards. (all bios modded, I used anorak tech bio's and then backed off clocks a little for card life with AMD catalyst wattman. I had been using the latest AMD drivers circa 12/31/2017 in compute mode. I was hashing between 29 and 30 for each card, they were nice and cool, and I was content. I am using unregistered win 10 Pro (an old version I own.. so I had to go through a bunch of updates upon install) On both of the rigs I right after I built them I was getting a windows message "task host is preventing background tasks" I youTubed a fix where you changed a HEX key value in start from 2 to 4, and this stopped it.
I toggled the PSUs Monday morning and came back this weekend, to maintenance the machines and trouble shoot.
First off, I updated bios double checked all my settings 4G encoding, Gen 2 etc. Unplugging all the cards, the pc booted right up no problems, I DL claymores latest miner, scrubbed Cayalyst drivers and DL the latest and started plugging in GPUs. 1 card no problem, 2 cards no problem, 3 cards machine slowing.. like 20 seconds of wading through mouse button clicks to switch cards to compute mode. 4 cards over a minute per mouse click... right click on screen (wait over a minute) left click AMD catalyst control center (wait 2 minutes) click gobal settings (wait 3 minutes)
At four cards installed, I started claymores.. no issues. Everything running ok.
Installing the 5th card, Windows became unusable. like 4 or 5 minutes between responses. sometimes freezing altogether.
I can switch to any three cards and all is ok, so I don't suspect riser card issues, but I have riser cards to swap out, maybe I'll try them.
Before this event, I had maybe a 12 second lag per screen switching to compute mode with 7 cards installed. I"m assuming this is normal?
I'm looking for advice,
thanks.