Same PSU, too? You can tell if they are using identical power with:
nvidia-smi --query-gpu=temperature.gpu,fan.speed,power.draw,power.limit --format=csv
Hope this helps.
Same PSU too
Are all the BIOS settings the same? Same on the PCI speeds, etc?
Same bios settings
lol ... going crazy ...
Removed all cards and restart rig with only 1 card direct connect to motherboard no risers,
Same low speed, 18-19 MH/s on eth
If everything is same and you swapped card from the "fast" rig and it perform worst on the "slow" rig it must be something wrong with the "slow" rig's motherboard or some bios settings are not exactly same. Are bios versions of the mobo's same?
On the "slow" rig reflash the bios, load bios default (or optimized) settings, save and reboot (F10), go into bios again and set everything same as on the "fast" mobo. Don't skip any options, open and check all settings are same. Take one GPU from the fast rig and try it. If it still underperform RMA the motherboard, I can't think of anything else.