When beching I used to get freezes during 3DMark (at random places). Try lowering the voltage feeding the RAM. Stock mvddc is 2.20. My card can bench at 999Mhz at mvddc 1.875 (maybe lower - haven't tried). 1.875 also fixed the random freezes during 3DMark.
As a bonus, card runs a lot cooler - suggesting that this freezes was due to overheating.
btw, Hynix datasheet says mvddc should be 2.20 for the 1.0ns and 1.1ns ram.
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