I got two U6 and both have the same problem - when mining, the frequency does not rise above 320 MHz. At the same time, consumption is 1400 watts and a speed of 140Gh. I got two D5 for $ 4000. 
I dont understand what could be the problem.
I dont understand what could be the problem.
Dont you use it in extremly cold or hot conditions?
What Is the chip temperature? My are around 70°C