What you really need to is ditch windows for Linux if you want to mine with that many cards on a single rig. Windows is unstable and tends to blow things up with random windows updates. Linux is so much more stable and is much easier to get a rig with that many cards running.
Turn off all the windows updates dude you will get the stablity in the windows platform too bro. I see many people complaining about the issue happening on the windows platform but there is no issues to me when I am mining the bitcoins.
You have find the issue resolved with the PCI riser cleaned and connected back. When the issue same happened to I have removed all the connected it back again to the mother board and it works well till now.