What is the cause of drop?
During GPRS transfer, there is no DL Power control. So basically, the amount of interference increases in the network if there is too much PS Traffic.
UL Power Control is usually not activated neither, so same thing apply in UL as well.
Can you see an increase in the ratio of Quality HO, or Interference HO (especially downlink ?)
You said the GPRS UL PC made it worse. How can GPRS UL PC impact the CS Call drop ?!? GPRS PC and voice PC are different mechanisms, with different parameters.
You can check also something (this is vendor dependent): the PS traffic is perhaps enjoying the best TRXs of the cell (less interference, less hopping, … ?) while the CS traffic is set on the worst TRX ?