Basically it's normal for them to crash from time to time. It depends how frequent however.

I would just lower your memory clock to like 1900Mhz and it will hash at 27.5 but it will be stable at least. Not all GPUs can hash at 29Mhs.

For your power consumption you should undervolt each GPU by 100mV and it should still be stable and consume much less power and be much more cooler.