the cause of this can be several but you can the below steps to confirm.
1. Check frequency in BBH and the retransmission rate in those cells. if high retransmission –> for sure there is problem on Frequency
2. Try use most BCCH TRX TS for PDCH.
3. Make sure (E)GPRS link adaptation is enabled.
4. If frequency change doesn;t get good result, limit the (E)GPRS coding scheme up to MCS7 instead of MCS9. MCS7 is more robust than MCS9.