Download Antminer-Z9-Mini-201805262047-500M.tar.gz from Bitmain support if you have batch 2 Antiminer Z9 mini. After updating it will show Sat May 26 20:42:30 CST 2018 for File System Version. Set frequency to 687M. I'm seeing 15.3 ~ 15.6 Ksol/s average. I set the fan to 75% as well. Running for 4+ hours atm with no issues. I believe the temperature of the Temp(Chip) has a lot with to do with the board dropping sol/s. I ran 700M at 65% in a 72 degree room yesterday for 3 hours but when I woke up the next morning only 1 of 3 boards were hashing. Will update status in the morning.