Rana showed up and diagnosed the problem as a railed FSS SLOW output.  The SLOW Monitor about was showing ~6V, which is apparently a bad mode-hoppy place for the NPRO.  Reducing the SLOW output brought things back into a good range which allowed the PMC to lock again.

In attempting to diagnose the problem I noticed that there is -100 mV DC coming out of the PMC RFPD RF output.  This is not good, probably indicating a problem, and was what I thought was the PMC lock issue for a while.    Need to look at the PMC RFPD RF output.