Since SCDCCH congestion has been excluded so the main focus is on air interface and abis interface. You have said that there is no problem after abis capture, my reccomendation are:
1. Please check quality of air interface e.g interference level, there is possibility that SDCCH loss due to bad quality of air interface.
2. Check access paramater e.g accmin, too low value accmin allows low signal MS to access the network. at this case there is possibility in SDCCH loss.
3. Move the SDCCh channel in better quality of TRX. As reference you can use measurement from mrr in ericsson.
4. from the SDCCH channel allocation (24 channel), i guess that there is a high load of using SDCCH (maybe it is caused by high LUP or high SMS or others), so please also do checking congestion on abis, due to implementation same feature such as abis multiplexing or abis concentration