I have similar problem with 1080Ti, only 20Mh/s at ETH, ETC (over DAG310). When mining other coins its all OK-50Mh/s (example Dubaicoin-DAG31).
I have just solved the problem by updating system to win 10 and use drivers from CD I got, because latest driver from NVIDIA doesn't work... so try this or win 7 regular update which doesn't work for me...