I’m not an expert, but I just want to tell 1 case I know. In ALU there is SDD TS(dynamic SDCCH) which can be converted between RTCH/SDCCH. If your cell has 5 SDD and your TCH traffic is heavy then(most of your SDCCH is converted to RTCH) SDCCH congestion may occur. If this is the case then try to change some of your SDD to static SDCCH (SDC ‘physical’-8 SDCCH ‘logical’). It may help.
If this is not your case I’m sorry – no idea.