Just got my batch 2 Mini yesterday. Ran it at 650 Mhz all night and had no HW errors and minimal pool rejects (using nanopool). Getting around 14.8Ksol/s. What's odd is I changed the cgminer.conf to set the speed to something higher but anything higher than 650 just gives me a really low hash rate, around 4.5Ksol/s, even 655 Mhz. No HW errors, just slow hashing. Anyone else experience this?
I noticed the Z9 mini seems to hash slowly unless you select speeds in specific increments. (usually 6 mhz) the only reason I know this is because after I experienced the same thing as you on my manually adjusted batch 2 mini, I looked at the OC options on my batch 1 mini and it looks like they usually let you select in increments of 6 or 7 mhz. I don't really know why but I'm guessing it's a memory timing thing. Try adjusting with the following exact increments until you get stability. 700, 693, 687, 681, 675, 668, 662, 650, etc.
Is that starting at 700 and going downwards or starting at 600 and going up? or does not make a difference?