If you sure that they work few minutes at 750 Sol then GPUs are 100% fine. First of all try downclock them to see what will happen. And as other ppl say, check temps, maybe you need to renew thermal grease.

I'm on equihash mining is about since zcash mining launch and used EWBF's miner since its second public version. So in my case I have original copy and I already found what's happening there. Devfee thread uses other pools than mine and when my pool get stuck with new work, devfee thread continues its work and hits much more than 2% of shares. And I found that sometimes it uses dev's account for shares that supposed to be mine and not marked as devfee, just try any kind of sniffer like wireshark etc. Those who say it's ok with original version, just did not notice that things. Anyway I still use this app but now I'm sure I need something to change about that.

The same thing is happening with my 4x1070 rig, something about once per two weeks. I thought it's happening when my pool has network connection issues, but can't say for sure. Sometimes I think that I must code miner myself if I had enough time. Really tired of unpredictable fees

ZEC looks promising for me, next ETH

I'd say best option is any equihash algo based coin, just look which one is reasonable for you right here right now.