I got this problem twice; In both cases when searching for maximum stable mem freq I can push the 1070 gpus.
Currently running stable at:
Core: -200 (minus 200)
Mem: 765
Power Limit: 59
Hashing: 32.770 MH/s
Do not understand how your 1070s work on these settings. With PL 59 for my 1070 hasrate is very not stable. When I setted MC 750, after some seconds I had 719 error and reboot. CC -200 also makes hashrate unstable. Now I'm testing PL +64, CC -100, MC +670 with power 750W and avg hashrate per card 32.06 (about 192.4 per 6 cards). So all my changes of previous OC settings is changing CC from +0 to -100.
Not all cards are the same, overclock potential will be different between models, individual chips etc so you basically need to be lucky aswell.