Is the site at the border of LAC/BSC?
If yes, then you need to try to change the re-selection Hysterisis.
If not, just check, whether :
1.SDCCH drops happen due to handover or not?
2.Frequency interference-CO-channel or adjacent channel (Check BCCH only if SFH, check also TCH if BBH)
3. THe TRX that brings SDCCH has a hardware failure.
Try to put SDCCH to another TRX.