regarding the timers, I have no experience. increasing them might reduce the drops but will increase the congestion. Here are defaults values:
T3101 = 3s Supervision of the Immediate Assignment procedure.
T3103 = 16.5s Supervision of the Internal Handover procedure.
T3107 = 14s Supervision of the Assignment procedure.
T3109 = 12s Supervision of the Channel Release procedure.
T9113 = 20s Supervision of the External Handover procedure in the target BSC.
If you decrease, then you might reduce congestion but might increase drops and failures. And vice-versa. That would be a good trial to attempt.
About the RADIOLINK_REP_DL_SACCH = 14, the default value is 10.
When you set it back to 10, is the Call Drop Level % going down ?
That’s a weird behaviour… it should not!
What are your values for :
RADIOLINK_RECOVERY_THRES (a.k.a N_BSTXPWR_M)
forget about REP_DL_SACCH_THRES (etc.) those parameters are statically set in the system. You cannot modify them. 3 and 10 are good values.