I assume you've tried resetting to factory defaults etc?
Yes several times.
But thanks for giving me a nudge to try it again, as Ive discovered what is happening.
I reset to defaults again and this time before rebooting I decided to have a look in the settings for the nb pcie config and low and behold there was settings there to be able to alter the gen version of all the slots, where usually I only had the option to change the gen of slot 1,2 and 4.
Saved and exited, benched and the results were even worse.
Rebooted, back in the bios into the nb pcie config and those settings had disappeared again and just left with the option to alter slot 1,2 and 4 again.
Something weird is going on.
So reset to defaults again, back into nb pcie config, those settings had re-appeared, This time before rebooting set slot 5 to gen 3.
Saved and exited, benched and the results were now as they should be.
Back in the bios, settings were gone again, but this time after a reboot, even though they have gone gen 3 has stayed.
Just have to figure out why the settings disappear now....lol