Are you sure the drop is corelated to the GPRS traffic ? What about CS Traffic ?
I would say that at the busy hour (CS and PS), then more radio frequencies are busy, therefore the quality of the air interface is degraded. That leads to higher call drop rate.
If that’s not the case, then let’s assume the GPRS traffic does indeed impact the CS drop. The only thing i can think about is how the capacity is distributed between CS and PS. Perhaps a CS call is trying to perform a HO towards a cell, but this cell can’t accept this incoming HO because all its resources are already occupied. And perhaps the PDCH are not released to make room for the incoming HO (this is a strategy which is vendor dependent).
In order to try out this method, try decreasing the max amount of pdch in the cells, and globally, try making the allocation of CS call more aggressive –> PDCH should be preempted faster.