Bell 206L Captain reported uncommanded changes of torque and yaw resulted in a controlled autorotation. Captain landed safely.

Narrative

At the time of the event the winds were out of 330 and at 6 knots. The sky was clear with unlimited visibility.The helicopter was flying in the left downwind leg of the traffic pattern to Runway XX at 1000 feet AGL. There were two other fixed wing aircraft in the traffic pattern. The helicopter's airspeed was 65 kts. and the torque was approximately 50%. There was a shudder in the airframe followed by an uncommanded rapid decrease in indicated torque to 20% then an equally rapid increase to 110% torque. There was also an audible rapid increase in rotor rpm. This took 1-2 seconds. At the same time there was a 10 degree uncommanded yaw to the right. I put the collective fully down; but there was no reduction of the indicated torque. I rolled the throttle to idle to correct the uncommanded over-torque and entered an autorotation. The helicopter yawed 30 degrees to the left associated with the commanded decrease in engine power. I turned the helicopter back towards midfield of the airport to have the most landing options. During the initial over-torque event until entering the autorotation; I allowed the airspeed to decrease from 65 to 50 knots. Once in the autorotation; I instinctively increased the collective slightly up to avoid overspeeding the rotor as I typically do in autorotation training. This allowed the rotor rpm to droop to approximately 95% which gave up available energy but extended the gliding distance.I advised on the CTAF and indicated I would be landing on Runway XX. It quickly became apparent that the glide couldn't be extended that far. At 500 feet AGL; I turned to a closer prepared surface; Taxiway 1. While in the turn; the low rotor RPM caution sounded that indicated 90% RPM. The collective was pushed to fully down again and the rotor RPM increased to more than 90% silencing the buzzer. The indicated airspeed further decreased to 45 knots during the maneuvering to the new landing spot. The airspeed continued to decay as I exchanged airspeed for rotor RPM while flying directly to the taxiway. The rotor RPM stayed over the low warning threshold until the landing spot.At about 100 feet; I aligned the helicopter with the taxiway on a heading of 310 into the prevailing wind. I flared over the taxiway to exchange forward velocity to rotor RPM. Collective was increased to use this additional rotor energy to cushion the landing as much as possible. The touchdown was typical of a full down autorotation landing.The engine was shut down normally via the throttle. The shutdown checklist was complete along with closing the fuel valve per the engine fire emergency checklist.