This morning I noticed that the first loop ISS went back to mis-behaving, for reasons unknown.
Attached are oscilloscope traces of the AOM control signal.  Whenever the first loop starts
oscillating, the AOM control signal behaves like this (scope_2.png).  A zoomed in trace is in
scope_3.png.

    scope_4.png shows the AOM control signal and the DC output of PDA.  The AOM driver is
deliberately turned off here.  The behaviour is present even when the light to the ISS PDs
is blocked.

    The ISS will be offline for a while, whilst I debug the board.