what is your MAX MCS ?
what is your GPRS HIGH LOAD ?
how many cell in the BTS ?
What is MAX TBF PER SPDCH (UL & DL) ?
Why only 2 Extra timeslots ?
so regarding the current info:
max pdch highload is way too low (try to set it to 7 or 8)
extra ts is too low
waiting for your answers,
cheers,
pix