I am suspecting a Riser seeing this, do some basic checking, like connections from USB to board PCI-e x1 or Riser x16slot to GPU if properly in place and doesn't have any loose connections. otherwise try to replace them if you have extras but ensure replacements is reliable and solidly working.
What is the best way to find a faulty riser/gpu or is there anything else which it could potentially be?
basically, you can do swapping/interchange/replacement method.
I see perhaps it's best to change all 4 risers and then do testing on them one by one. this will eliminate downtime on rig. I'll await till my new batch of risers arrives. Thanks!