Ok Gerald.I could have given u the exact solution,but for that i would require some more inputs,So U can Try any of these or all of these solutions one by one
1.U can Try to reduce the RACH Min. Access Threshold & RACH Busy Threshold to overcome this problem.
2.Please check if There is More LOC Update request in that cell.If there is many LOC update,u can increase the SDCCH.
3.Try remove additional SDCCH’s if any and try to put Only One SDCCH/TRX.
4.Make the BCCH TRX Lapd of 32kbps.
5.Check for Co-BSIC-BCCH,if any nearby.
7.Also check if there is Channel Activation Nack for this cell.