S9i hashboard, sometimes pass the single-board test, sometimes report 0.  

Whenever it reports a 0, simply measuring the voltage at the IO ports' Tx and Rx pins ensures it will pass. All chips that can be measured with RI are present and functioning properly. What could be the situation here?

Re:

The issue may be the IO port on the hashboard where the ribbon cable is connected. 

Please check the surrounding components and materials for any potential problems.