RX470 with low hashrate - How to fix?

[swellcoins]

I have a rig with 6 AMD RX470, Asus Prime Z270-A Motherboard and Windows Pro 10 (64). I am gettin 19.8 Mh/s per card mining ethereum. My other RX470 rig gets 21.9 Mh/s. Same cards, same brand, different hash rates.

There is something wrong with the 19.8 Mh/s rig, because Claymore does not report the temperature and fan velocity of the cards. Can't figure out where is the problem. Also MSI Afterburner does not work on this rig. Card driver is 171.1024. The rig with 21.9 Mh/z hashrate has Driver 162.4

Can the problem be in the card driver? If so, how do I uninstall and install the correct driver?