I have the same cards and same problem.

The solution I found is that running the rigs headless, i.e, without connecting the monitor. The important part is that when I turn on the rigs with a monitor connected, hashrates drop about 100-150 kh/s per card even if I disconnect the monitor later. But if I disconnect the monitor then reboot the machine via ssh (I use linux) every card produces full hashrates they are capable of. I have no explanation for this behaviour but all 3 of my rigs occasionally do it and I can consistently solve it with procedure I described above.