PRMI glitch certainly comes from power recylcing gain fluctuation.
I confirmed this by
  - Reading the value of POPDC at the time when there's glitch in error signals
      -> There was some threshold for POPDC to make a glitch
  - Look closer to the glitch
      -> It was oscillation in ~400Hz, where we have phase flip in PRCL/MICH servo

Next is to find why we have power recycling gain fluctuation. I want to see the correlation between alignment fluctuation of optics and POPDC.

Glitch analysis:
  Below is the plot of
   Red   PRCL error signal (C1:LSC-REFL33_I_ERR)
   Green MICH erorr signal (C1:LSC-AS55_Q_ERR)
   Blue  PRC intra-cavity power (C1:LSC-POPDC_OUT)
  when PRMI is carrier locked.



  Time when there is a glitch in error signal is marked. Value of POPDC at that time is also marked. It looks like there's some threshold (dotted blue line).
  It sometimes doesn't show glitch even if POPDC is above the "threshold". It is maybe because of alignment fluctuation. Intra-cavity power gets high, but power at PDs get low, or vice versa.

  Right plot is closer look. Glitch is a sudden oscillation at ~400 Hz. It is the frequency where we have phase flip in PRCL/MICH openloop transfer function now(see elog #6950).