You must point that 570 is Sapphire Nitro+. It's good quality cards. On not so good cards like MSI Gaming X (not plus) 2100MHz isn't stable on doctor's strap. Max 2075-2085MHz and it's only 950-960 h/s gives. On 2100MHz comp hangs after few minutes same as 2150, wich hang comp nearly after launch of miner.
Must say - anyway doctor's strap "999..." is a little better than "777..." strap. It gives max 930 h/s on 2100-2125MHz.

P.S. I think that doctor's strap can be improved for stability to reach more memory speed on not good quality cards, but it'll lower hashrate... It's need to work on it and test.

Doctor, can you modify your strap a little for improve stability? At least say what it's need to change and I can test it on MSI.