Gminer isn't working fine on multigpu rig with 5 1070 on GRIN. When I run 1 instance of miner I get 1.2 g/s per card and 6 g/s total and 5-6% CPU load. When I run 5 instances - 1 per card - I get 2.5 from every card and 25-30% CPU load
I guess you have used Windows 7, right? I saw same problem on Windows 7, bminer has same problem, I think there is some problem with driver version/OS.