On the L39 C/ ZA the engine used has a bleed air that operates in between 75 and 85 percent HPC RPM so you will see it fluctuate rpm in this range so its normal and correctly modeled. The correct procedure i have found is for a long leg approach to use your airbrakes adn set the rpm just above 70% and use your airbrakes as you decend to lower your airspeed below 300ias. Then while managing rpm below 300ias you lower landing gear T/O flaps and then 100m short of the runway your LDG flaps. The second alternative is to keep engine rpm above 85% and come in for an overhead break to bleed airspeed that way then follow landing checklist again. But yeah dont keep the rpm in between 75 and 85 percent as the bleed air valve continually opens and closes.