it sounds like there is a misconfig in the MSC.
the SDCCH drop is caused by the expiry of a timer during the sdcch phase. The sdcch phase is a dialog between MS and MSC : authentication, ciphering, call setup, equipment check (= DTAP messages).
Timer expiry is normally caused by a radio failure, but in your case it might be due to a non-response from MSC or from MS.
Have you performed a A trace to detect when is the failure occuring ?