I had the same issue with mine with the icetower and noctua fan. found the sweet spot for it at 600/1100. getting 1.3TH/s pretty consistently with it now
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Would you look at that. I think it worked!! 
glad to see!
the benchmarks chart on this page was helpful for me https://www.solosatoshi.com/how-to-overclock-bitaxe-gamma/