1.There will be PS assignment reject or failure in this case.Have you checked the reasons.
2.What is the Rx_lev_acccess min for GPRS.check that.
3.Is there N3101 or N3103 overflow.
4.If call setup indication rate is poor.In this case,check the PS paging success rate LAC and RAC wise.
5.Are you using NACC or NCCR(Network controlled cell reslection)?
6.What is the value of t3168?
7.Are you using extended UL TBF function.