Here is one solution from the bitmain troubleshooting website:

9.Issue: The miner works with normal hash rate when it was booted firstly. After a while, there is no hash rate.

Reason: One of the fans was not detected. Only when two fans are detected can miner work well.

Solution: Switch the fan and IO board in turn to find out which one is defective.

I'm trying this now... this suggestion came from bitmain's troubleshooting website: https://bitmainhelp.zendesk.com/hc/en-us/articles/220872007-Possible-problems-for-Antminer-Troubleshooting-for-S7-S9-L3-