ok, sounds like real radio issue on GPRS. You should focus on decreasing the Coding Scheme (CS), in order to make it more robust.
1/ CS-INIT = CS-1 rather than CS-2
2/ LINK ADAPTATION (LA) = ON
3/ Is your EGPRS activated (or GPRS only) ?
4/ Experiment increasing OR decreasing the PDCH INACTIVITY TIMERS. I’m not sure which one will actually decrease your TBF DROP% the most.
5/Ensure you allow 4 PDCH or 5 PDCH per TBF
6/ Maybe decrease the MAXIMUM CODING SCHEME (MAX_CS) to CS-3 rather than CS-4
7/ Prefer the TBF on BCCH FREQ (you should find such settings somewhere, which will favour tbf establishment on BCCH freq until it’s full, then the next estab will be on non-BCCH freq)
That’s about it, on top of my head.
Please let me know how it goes. Don’t forget to tell me if EGPRS (EDGE) is activated or not (MCS-1 to MCS-9)