@Claymore,

Could you please check memory controller load? As others stated some cards randomly gets lower load on memory controller and this causes decreased hashrate.

For instance, my average hash rate should be around 264,5 MH/s for my 9 gpu rig. But it displays effective hourly rate around 205.0 MH/s.

I have tested this for more than 7 hours and this is the case for me. Even on ETH only mining using fine tuning and low power bios.