Well, when I was playing with mining in the beginning with one GPU (#1), I get used to overclock it with +X core +Y mem.

When I got my second GPU (#2), applied the same +X core +Y mem. The miner was mining for a few seconds before the GPU #2 crash (0 hashrate or crash the miner).
I tought "Ok, this GPU can't overclock like the other" and decrease a bit the overclock of the GPU #2 and stop happen. But then noticed that it was happen to the GPU #1 some minutes after the miner start and had to decrease the overclock of GPU #1 also and never happened again

My conclusion: One GPU alone is more stable with their overclock than working with others. That may causing your mining rigs with more GPU to crash or get some 0 hashrates, maybe you have to decrease the overclock of yours GPUs when using more GPUs per rig.

The BUT: I can't say with it happen only to [1 GPU -> multiple GPUs] or [1 GPU  > 2 GPU > 3 GPU > 4 GPU > ...] since I only tested with 1 -> 2 GPUs.