I second that. When overclocking settings are too high, I get "NVML error: cannot get temperature" and "NVML error: cannot get fan speed" in Claymore's log file, and the hashrate becomes null on that GPU.

chanyewhau,
Ethash algorithm is more memory-intensive than Equihash and memory instability manifests itself faster. What is the exact memory frequency - check GPU-Z or another Afterburner skin, that shows the actual frequency? Try to run the miner at stock clocks and PL 80. Sometimes it happens that cards do not hold even a 10% clock increase.
BTW, what memory size and type is installed on your GPUs?
Fans running at 100% will not shorten the life of your GPU, though the fans might fail sooner due to increased mechanical stress (mainly the bearings run dry and might jam).