DIAGNOSIS The high-speed circuit in the carburetor is clogged or out of adjustment. It's getting enough fuel for low RPMs but not for the high-speed demand.
FIX Start with the carb adjustment screws. With the saw warmed up, slowly turn the high-speed screw (H) counter-clockwise in 1/8-turn increments while gently blipping the throttle. You're listening for a point where it revs up cleanly without hesitation. Don't go more than a full turn out from the baseline setting.
IF THAT DIDN'T WORK The fuel filter in the tank might be partially clogged, or the carburetor diaphragm might be stiff and not pumping enough fuel. A carb rebuild kit is the next step.
HEADS UP Adjusting the H screw too lean (too far clockwise) will make the saw over-rev and risk serious engine damage from overheating. If it sounds really screamy, turn it back out.