You seriously using this parameters?
1150/850
2150/875

Because 850mV for the core is quite low at this speed, also 875mV for the mem with 2150 is also quite low.
Are you sure you are not getting HW errors, I mean don't look at HWiNFO64 because it is not showing all errors.
I had also no errors with HWiNFO64 but the cards stopped working from time to time without showing some errors in the HWiNFO64  software.