2.5 hours into being locked at 50 W, we saw two PI ring up: ITMX 15522 Hz (Mode 2) and ETMX 15541 (Mode 17). We couldn't damp them and they broke the lock (22:22 UTC). 

We had about 10 minutes before lockloss to try different damp settings. For Mode 2, any phase/gain changes I made rang up the mode more; the existing settings were already providing the most shallow slope. For Mode 17, a slight phase change helped but instability still bounced back. Below is Mode RMS monitor after the struggle. (Green and Red modes only appear to be ringing up from bleed over).

I've also attached the trend of the two modes. 

Something looks like it really kicked on Mode 2; there's strong slope changes before we started trying to adjust damp settings. Both modes stayed rung up into the next lock aquisition which I've never seen before; we could see them on the RMS monitor before DC READOUT. I stopped at 30 W to tweak phase settings again and allow to damp. These settings have been saved to the guardian. After that we continued to 50 W and stayed locked for almost 2 hours without incident (lockloss unrelated to PI). Overall, after tweaking at 30 W for full peak minimalization, I've lowered gains (perhaps the high gains were 'kicking' things too much?). 

This is the same time scale into the lock as we saw the other night, though note that ETMX did not ring up and ITMX did not have any active damping that night. Also note the oscillation in Mode 17 ~1156310300 in the trend.

We're leaving the interferometer locked at 50 W with the tweaked PI settings so we'll see in the morning.