If there is no high voice, SMS or GPRS traffic, no interference, if its not a LAC border and the neighbour cells are fine. There is no reason for SDCCH congestion
There are a lot of parameters in siemens that impact on the immediate assigment procedure. But it seems ( this is a hunch) that you are experimenting configuration problems. You should contact your vendor or try a drive test and see what you find in there. Please let me know how you have solved the problem