Witnesses observed the helicopter flying northbound while traveling parallel and just east of a north-south highway. Reportedly, it entered an autorotative landing approach, crossed over the highway, and impacted in an open field on a westerly heading after clearing power lines, then burned. Witnesses reported that the helicopter was descending at about a 45 degree angle when it went out of sight behind trees. An investigation revealed that it impacted in a nose-high attitude while moving to the left. The wind was from 340 degrees at 16 knots. After impact, the helicopter began to disintegrate. Wreckage was found over a distance of 108 feet. During an engine teardown, damage was noted that was consistent with a lack of power and low RPM. No other evidence of preimpact malfunction or failure was found during the teardown. Reportedly, the pilot had noted vibrations on the previous flight during left turns and had to "beep up" the engine after lifting off to a hover, then had to "beep down" after landing to keep from overspeeding.