I was trying to lock and look at the ASS for the Yarm, but the transmitted power was oscillating very near 1Hz. Eventually I looked at the mode cleaner, and it was also moving around at a similar frequency. I took spectra of the ETMY SUS damping feedback signals, and they (POS, PIT, YAW) saw this 1Hz motion too (see attached plots...same data, one is a zoom around 1Hz).
As a first place to start, I turned off the WFS, which immediately stopped the MC oscillation. Turning the WFS back on, the oscillation didn't come back. I'm not sure what happened to make the WFS bad, but I perhaps had the ASS dither lines on (I've had them on and off, so I'm not sure), although turning off the dither lines didn't make the WFS any better.
As an aside, the MC refl with the WFS off was ~1.5, rather than the ~0.5 with the WFS on, which means that the PSL beam and the MC axis are not well matched.