I have Antminer S9 (13.5GH/S). Sometimes the hash rate of one hash board falls very low.
http://ipic.su/img/img7/fs/Untitled.1518701512.png

Two other boards always work stably (about 4500 GH/S).
There is no “X” shown on the ASIC status.
I have updated firmware to the latest version.
http://ipic.su/img/img7/fs/Untitled2.1518507242.png

Reboot helps but only for a short time.
What could be the reason?