Im not familiar with Siemens PCU but in other vendors a PCU load of 80% is rather high ( and should be investigated as a possible cause of your problem)
what kind of services do you deliver (wap, ptt, internet)?
For only wap services with that amount of PDCHs (2+4) dedicated to GPRS you shouldnt have problems of TBF establishment but if you deliver ptt services 2 PDCHs + high voice traffic can cause congestion issues ( but this would be an odd situation)
One test that you can make to discard BTS capacity issues is to increase the sharing of PDCHs for different TBFs? if you change this parameter and de blocking decreases you need to add more PDCHs ( this will tend reduce the throughput but works as a test)
Do you have EDGE or CS3 and CS4 in your system? how they perform?