I have 6 1070's on a rig with h81 pro btc and g1820. After 15-30min the cpu goes to 100% and the rig becomes unresponsive. Some of the cards start mining with 4-5mh and I start to get rejected shares. I'm using MSI AB to overclock my cards. Task manager reports System, System interrupts, EthDcrMiner64.exe or MSI AB maxing out the cpu.
A few people have been having similar issues - myself included. Haven't managed to find a solution yet. It might be hardware related, like insufficient ram for that many GPU's to maintain being run at max capacity from straps? Only speculating.
In short though, I'm in the same position as you and if I find a solution I'll post here or PM you.