You say you have plenty of power, which 3400 W is plenty, but you fail to say how it is being supplied. Is it one big server PSU, a server PSU and a normal desktop PSU, a couple of desktop PSUs?

From the limited information given, my suspicion is your problem is power related for the simple fact the Zcash algorithm usually draw a bit more power than mining Ethereum. So depending on how your cards and risers are being fed, maybe that little bit extra power draw is causing your headaches.