1) Cross-talk is possible on Air Interface too as someone explained(when ciphering deactivated).Make Sure your MO authentication is activated when u enable ciphering.
2) Make Sure your BCCH and BSIC combination is Unique.
3) Check with ur Trau modules(Code) Maye be the PCM Timeslot is Overwritten.
4)CIC -mismatch can be a reason too.