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.