WOOT! after 2 days of pulling my hair out and f&%*ing around, I have solved my problem. It was power distribution. The rig I was testing on has 1 750W power supply and a secondary 1200W server supply. I was stupidly powering a 1080 and 2 x 1070s on the 750W supply and only 1 x 1070 on the secondary. No wonder it was rebooting!

So now I am a Cryptodredge convert. Thank you Cryptodredge devs for a really great miner. I have an improvement of at least 20% mining cryptonight-heavy algo.

1070 cryptonight-heavy XMR-Stak 720-760 h/s  CryptoDredge 870-900 h/s

1080 cryptonight-heavy XMR-Stak 670-690 h/s  CryptoDredge 805-810 h/s

I can also easily run a cpu miner concurrently (which I wanted to achieve because XMR-Stak uses both CPU and GPUs) and I get another 150 h/s.

The whole rig measures around 500W. So that is about 3.6 kh/s for 500W. I wonder if those AMD vega miners can do that well? Also, XMR-Stak has a 2% dev fee (unless you compile yourself) and CryptoDredge is only 1%.

I think CryptyoDredge must officially be the best cryptonight variants miner for Nvidia GPUs. Thanks again devs - your miner is awesome!