We are back to Observing but I am continuing to monitor the pesky PI mode 26.  I worked with Terra over the phone for ~30 minutes and we were able to determine that the BP filter setting were correct for the current mode frequency (15010 Hz).  I had changed them earlier from 15009 (and accepted the SDF diffs for this mode), which according to Terra should have been close enough to be effective.  We were able to get the mode to slow its ringing up, but were unable to damp it down.  Currently it is ringing up slowly and I expect it will break the lock again in an hour or so.  I'm not sure what else I can do about this mode as all the knobs have been turned and double checked by 2 sets of eyes.  At Terra's suggestion, I'll keep fiddling with gains and phases in hopes of stumbling upon the magic combination.  Neither of us has seen this behavior with a PI mode before.