99.9% of gpu frequency, hashrate and any other drop has to do with power, 0.01% has to do with memory errors. So check your voltage correct and memory errors and more importantly your psu.
Thanks.
I checked, my PSU is fine. These cards are really tricky to setup, they are behaving strange, everything I do through AB to reduce power consum. will result in instability. Don't really know since this can happen 2 hours from running miner or one week after.
Regards.
I've found cause of my MHz(hashrate) drop. I have water pump in the house that occasionally turns on to fill water in(house water system) when certain level of pressure is reached and it does it couple of times a day. I noticed that pump is using much power(draws pretty much electric current) in the moment of ignition which caused the instability of my PSU in a way not keeping correct voltage on GPUs. So i decided to turn it of for a couple of days and voila, got stable mining, no problem at all.
Thanks for advice to check power, you said that it is 99.9% power issue, and it was.
Best regards.