The JetRanger and the OH-58 may have similar silhouettes but they are two very different beasts for two very different purposes. The former is built and operate to military standards while the latter is built, certified and operated to civilian standards. It therefore follows that you will find differences - sometimes significant differences - in the respective flight manuals.
Having said that, you say you fly an OH-58 and therefore you should be using the flight manual for this type - ignore the JetRanger flight manual limitations.
The problems you describe regarding main rotor over- and under-speeding would certainly seem to indicate a problem with the power turbine governor and/or the N2 rigging - this should be the area of focus to start your troubleshooting.
As
Ascend Charlie has said, first determine if your engine has a Bendix or Chandler Evans fuel control system. If it's a CE system ensure that the PTG is properly bled.
For both systems check for slop/backlash in the entire N2 linkage and ensure the rigging is correct. In a properly rigged and functioning system the over- and under-speeding of the main rotor should be minimal - despite flying the helicopter "rather aggressively". Let us know what you find.
