First, you must ensure the GSM QoS is good in the cell. Basically, ensure the radio part is reliable (no interference, neighbors defined, rxlev > -85dBm at the cell border, etc.)
Once this is done, it means your cell has a good GSM quality of service, but a poor GPRS qos. So you should check that your cell is not preempting some of the PDCH to voice calls (leading to TBF drops), or that your MFS/SGSN/GGSN are all working correctly.
And no congestion on Ater and Gb interfaces.
Of course, you should put all your radio parameters regarding GPRS to their default values. (no UL power control for instance)