You’re using base band hopping, right?
in BBH Tch in use is moving from TRX to TRX. SACCH are from FACH which are stolen for TCH so they are moving to. Of course all what can interfer with TCH (co-chan, interference) will interfer with SACCH. which can be a first point for investigation.
You also have strong dependencies with Antennas system. If for any reason antenna path on TRX1 has less loss than antenna path on TRX2 then there will be TCH effect (so SACH effect) .
What i’ll do is stop frequency hopping and try to find any HW issue (TCH losses, BER..)