I followed up the range drop tonight at 2015/06/07 around 8:30 UTC.

Here are the symptoms:
 - 25.4Hz and harmonics ringing up and saturating everything.
 - Significant increase of two lines: 842.781Hz and 868.188Hz (Yes, they are 25.407Hz apart) Those are marked with crosses in the attached plots.

Note that there are some other lines that increased (red) over the reference (black), but they are symmetric 25.4Hz sidebands of strong lines that did not increase. The two lines at 42.781Hz and 868.188Hz are not such modulation sidebands.

Parametric Instability?
alog 17903 reports on an observed parametric instability at 15540.6Hz, causing a line at 843.4Hz. Seems close enough to suspect it as the culprit for 842.781Hz. But what is 868.188Hz? And why does the 25.4Hz show up so strong?

BS roll mode?
My first though on seeing something at 25Hz was BS roll. But T1200415 reports the BS roll mode at 25.9715Hz... If we believe that, 25.4Hz can't be the BS roll...
Do we have an actual recent measurement of the BS roll?
I did try to look at the BS oplevs for a sign of the roll mode rung up - nothing. I haven't looked at the OSEMS yet

All attached plots were taken at 8:15 UTC, just before it go really bad. The black reference is from 7:00 UTC.

I also left instructions with Cheryl on how to lower power if this happens again. If that fixes it it would nail the PI.

Running of to the airport, but Cheryl will follow up.