I also have these cards, have finally got them running stable @ 1.1v ~720Kh/s
Using almost the same settings as in your post, however I found that these cards don't like anything over 1050 GPU engine
My settings:
--intensity 13 -w 256 -g 2 --lookup-gap 2 --thread-concurrency 8192 --temp-target 78 --auto-fan --gpu-engine 1050 --gpu-memclock 1500
I already tried those settings with same voltage, pc shuts down after 2 min mining.