I would:

1. look at the free swingin michelson. Should be able tu null that siggnal in all ports to define the Q phase.
2. If things are weird, put an RF signal nto the demod board mhich is offset from the LO by ~100 Hz and verify the demod/whitening chain is kosher.
3. Lock PRMI and drive lines > 200 Hz. If PRC/MICH are not orthogonal, then there may be a mis tuning of RF SB wavelength and cavity lengths.
4. IF PRMI is sort of healthy, we could be having a weird SB resonance in the arms.