Thank you for you suggestion. I live in Thailand. After a few attemps to upgrade the lastest firmware, the average speed increased to 12,500 GH/s. But I'm not sure that the speed will remain after restart, so I'm not dare to do it. As you can see, the chips are not really die, instead it seems like a software problem. The shipping cost to bitmain china would be around $35, but I don't want to risk lossing or damaging the board. I have emailed to bitmain support, but no any response yet except the autogenereated reply.

It is not usual "x" everybody know. It will not dissapear easily by disconnecting power. For now I lost 100 GH/s in total. Mine is batch 6 12.93 GH/s model. 

This is the screenshot.
https://imgur.com/a/hxGCF