sorry, i can’t help about that. I suggest an A interface trace, perhaps you can detect during which message there is a failure.
Sdcch phase mostly contain “DTAP” messages, such as Authentication, TMSI reallocation, Ciphering, etc. They all require an action from the MSC. If the MSC is unable to answer, then the SDCCH becomes unactive and eventually drops.
But it happens on only 1.2% of sdcch, so it sounds random… however, i’d check the a interface and MSC configuration first.