ldisrr means the DL TBF has dropped due to a radio timer expiry (whichever one of them).
so I would focus on the radio quality of the TRX which carries the TBFs, the BCCH trx usually.
Ensure that there is no new cell in the area, using a coBCCH or adjBCCH.
Then ensure that the CRH and CRO and ACCMIN are not set too high compared to the direct neighbours.
Ensure the CBQ is at the same value as the direct neighbours.
Ensure the neighbours cell BCCH are correctly present in the BA list IDLE and ACTIVE.
Ensure LA (Link Adaptation) is activated, and LQC (for EGPRS) is activated too.