hi
the SDCCH congestion may be due to the following cases :
1/ not enaugh SDcch time slot recommanded value is one SD/TRX .
2/ the cell is border so there is a lot of location up date ; check CRH and T3212 timer
3/ BCCH type combiner or not because there is two kind of SDCCH:
SDCCH/4 ( 4 sud channels ) SDCCH/8 (8 sub channels )
4/chech if there is a SMS traffic in this area