when failure is 100%, the problem always come from a faulty HW.
Now, i don’t know Nokia systems, but you might want to check the PCU (packet control unit) congestion or processor overload. Under extreme stress, I assume defense mechanisms are activated (tbf assign reject?). Also, maybe it’s your BTS processing unit that’s overloaded.
I have no other suggestions… check interfaces congestions as well (ater, abis, Gb)
I don’t think the core network (SGSN) could be the problem here.
Again, i would say that’s either a BTS or a PCU HW problem.