Den noticed that the -15V LED on the TTFSS board was not illuminated. A further symptom of the FSS being funny was that the PC RMS Drive was constantly high (3.6 ish) and the FAST Monitor was very high, often saturated.
We took the TTFSS board out, and put an extender card in, and it looked like all of the correct power is being supplied to the board (the +\- 24V LEDs on the extender card were illuminated). Just to check, we put the board back in, and this time both +\- 15V LEDs came on. So it looks like the board is fine, it just wasn't seated in there all the way.
Now the readbacks on the FSS screen look good (PC RMS Drive is less than 1, FAST Mon is 5ish), the MC is locked, and I think we're back in business.