Serg,
It is difficult to understand your text. Are you doing an automatic translation ?
Back to GPRS : the problem comes from radio quality. The quality can be bad for GPRS connection and be good for Voice connection, there is no paradox here.
Check your RC retransmission rate, check your MCS usage, check your TBF Drop rate, their causes.
Check the TCH Establishment failure causes.
Regards
pix