I just received my first second batch Z9 mini too. I had 2 from the first batch both running at 750M (16.2 and 16.5 KH/s)without any problem since day 1 with fan to auto.
But my last one from batch 2 has the same exact problem as many here. I'm kind of happy to have found this thread and to realize I'm not the only one. I also thought that 681 was the max OC but after a few hours I lost a board going to zero. I am trying at the default 500M to see if the problem occurs too.
So Bitmain made sure they put the very best board in batch ones so they get some nice reviews all accross the net to sell more with batch 2 where they dumped all their faulty boards... And they will say... well only 10Khas/s is garanteed folks!
I hope I'm hallucinating and someone will just find a quick fix!