I switched to nicehash, numbers are (unfortunatelly) same as before, no change at all 
I have added screenshots showing max and min avg hashrate in 17 hours
https://imgur.com/a/NtomIlz
https://imgur.com/a/zOgfak3
https://imgur.com/a/paw90gb
I have added screenshots showing max and min avg hashrate in 17 hours
https://imgur.com/a/NtomIlz
https://imgur.com/a/zOgfak3
https://imgur.com/a/paw90gb
Cool thank you very much.
it seems that this miner just dont like temp. over 50 degrees, in first test fans were moving at 100% to keep temperatures below 50,
I transferred miner to the original colder place where it was before and run tests again, here are the results:
original V1.7 firmware on nicehash
https://imgur.com/a/Wgw24hQ
new V1.8 firmware
https://imgur.com/a/nU1TpAV
strong u6 v1.8 hashrate on nicehash
https://imgur.com/a/CMRuKkl
difference is more than 20 GHs and so far no problems, but there could be in the summer