I had exactly the same problem, 37.30MH with Mem OC of 1150. One single MH beyond that and hashrate dropped. Until I found out the mem chips surounding the GPU are not in contact with the copper cooler, basically, they are just cooled by the air flow passing over them! A horrible design flaw. In may case, it’s a INNO3D. So I bought some thermal pad and put it on the chips. Even so, one of the mem chips row are so far form the copper, like 8mm, it’s impossible to make it be in contact with metal, see the picture.
In any case, after that I’ve been able to OC it up to 1600, though it only reaches 38.3MH. Setting the fan to highest revs increases the HR up to 39, so the problem is clearly the temperature of the memory, which is not the temp shown in HiveOS, that’s the GPU