MD500 pilot encounters engine problem and executes forced landing.

Narrative

While conducting external cargo work with a 30 ft. external cargo suspended 80 feet from the belly hook of a MD500-D; I experienced a substantial power loss twice resulting in a jettisoned load and a forced landing. I was approaching to make a pass on a lightly wooded area when at an altitude of approximately 180-160 feet and 20kts of forward speed; the aircraft rpm dropped below 98% NR; yawed rapidly; the engine auto relight engaged and the low rotor warning activated. As I was looking out the right chin bubble and moving away from the wires to prepare to execute an autorotation landing into the trees; the rpm returned in a surging manner. I was able to begin climbing and obtain some forward speed when the rpm again dropped with a more dramatic yaw than before and returned partially. A check showed the engine was still making relatively good temperature however the rpm was not enough to maintain flight. I notified my ground man that I was headed back toward the landing zone with an emergency and upon reaching a clear hay field a few hundred yards down the line; I punched off external cargo. With the reduced rpm I was able to milk the collective lever and land under partial power in the same hay field approximately a quarter of a mile from the landing zone. There was no damage to any person(s)or property on the ground or to the aircraft however the cargo was destroyed. The company's emergency action plan was activated; notifying the company's Owner; Chief Pilot; Director of Maintenance; and Area Coordinator. The next day; the company Mechanic arrived and after inspecting; diagnosing; and replacing the fuel control and governor the aircraft was returned to service.