On June 4, 1995, at 1800 central daylight time, a Hiller UH-12E helicopter, N4254X, was substantially damaged during a forced landing near Checotah, Oklahoma. The airline transport rated pilot received minor injuries. The helicopter was being operated by Main Street Aviation under Title 14 CFR Part 137. Visual meteorological conditions prevailed and no flight plan was filed. On the enclosed Pilot/Operator Report, the pilot reported the following information. While dispensing a herbicide, "the engine lost power, RPM, and a vibration was felt. The engine recovered to full power causing the helicopter to yaw." This event occurred three additional times during an attempted run-on landing to a field. As the helicopter's skids contacted the ground, the pilot heard a "boom", the helicopter yawed, and the skids separated from the fuselage. During the ensuing landing, the helicopter came to rest on it's side. The pilot reported to the investigator-in-charge, that an examination of the helicopter revealed the following additional damage. The tail rotor gear box was separated from the tailboom, and the first section of the tail rotor drive shaft was separated from the drive shaft and main transmission. He further reported that the reason for the erratic running engine had not been determined.