It doesn't make sense to me, but I'm far from an expert on GPU architecture. It seems that if a card is happy for months at 1125/1500 with I19 and 1 thread then it wouldn't suddenly become sick/hang/die at the same clocks (or even a reduced memory clock of 1250) with the intensity vastly reduced but 2 threads. I'd always thought that hard hangs were caused by too-aggressive overclock settings. Given that out of 60 identical make and model GPUs it's always the same 2 that hard hang, it has to be something about the specific hardware, but then if the ASIC quality was low I'd expect problems at 1125/1500 regardless of intensity or thread count.

My head hurts    I think I'll just put the two problem children in the same rig so that at least only one rig has a non-standard config.