From a Model 250 repair facility.
The majority of C20R surging/stalling can be atributed to the Turbine stage 1 nozzle.
The original Pt No 23038219 and then the replacement MAR-M version, Pt No 23062754 caused choking at the Turbine inlet. Rolls Royce agreed to us fitting the C20B engine stage 1 nozzle (Pt No 23062753) with a different flow area and this is just about standard now at all the RR approved facilities and has almost eliminated the problem. Also, to get the optimum performance, the C20B type stage 1 nozzle should be installed if you embody the power turbine enhancement.
One other area to look at is to have the Compressor bleed valve closing adjusted to the engine requirements, rather than the bleed valve test rig setting.