As others have said about their batch 2 miners, the third board on my batch 2 miner just doesn't handle OC very well. The other two boards seem to do fine with the higher numbers but that third one either doesn't hash at all or does so very poorly. I have to limit the OC on the whole thing to accommodate that board.
Same for me. Batch 1 has been going strong at 750 for a month but the third board on my Batch 2 won't let me get to 700. Currently running at 687 for the time being. It took a few hours for the third board to crap out at 700 so I'm still not sure 687 is going to hold.