Anybody know what I can do to improve performance in my rig

Have 3 x RX570 4GB.  Am running v 11.2 with the -h set to 896.
Card is clocked to 1350 (1250 is standard) and the miner never seems to get it to rin over the 1200Mhz range.
I chance the -h to 1400 and increase the Mem clock on the card but this just crashed the rig.
There are not other options set in my config file.
I'm getting 670 H/s on each card currently.

I have the latest blockchain drivers installed, catalyst 15.2 and afterburner.

Thanks