I think I've noticed a significant drop in 1 gpu hashrate during mining. This is the 2nd time I notice this happening, looking forward to understand the real cause and solution if possible.

This rig is currently now 6-gpu rig, it was 5 before, every of them run smoothly and after I inserted an additional GTX 970 yesterday the result was satisfactory at first! Around 270 sols/s for 970 using EFWB Zec miner.

After a restart the hashrate of this card strangely drop to 220 and never goes up again. I can't figure this out? Have anyone experienced this before and what is your solution?

PS. This also happened with my old rig of 4 gpus (different set of cards) however I didn't fix it as I think it might be a glitch, but this time I think I'll need to get it fixed, otherwise the performance per watt will be real bad for 970 that draws huge amount of power.

Thanks