i have that too with my 5x 470's they run average 760hs but 1 or 2 of them run in the 400-500's
i have to restart the miner a lot of time and change a bit clock setting till they all hit 750+ than i reset my clock to 1920
How are you hashing that high with those? Custom BIOS? I'm hashing 640 on 480 8GB cards and dream of achieving that.
Also, I would like to confirm that I had fan issues as well. If my fans were on auto, two of the six cards would go at a higher fan rate and the other would stay the same. Another issue that I've had was those same two cards that would fluctuate fan speed, would sometimes not have their fans spinning, and hashing at 100%. I have figured out that it was MSI Afterburner causing the issues with the fans. Once I removed AB, fans now are auto, but don't fluctuate up and down. I am now using Wattman to OC/UV.
Claymore 9.6/9.7 had these issues. Driver is 16.9.2. Win 10. Did test next driver up, same issue, but have not gone further with updates.
I have also 6x rx 480 8gb cards and they hash around 640-690 hash per card. Power consumption per rig 700w.