A PARTIAL LOSS OF ENGINE POWER DUE TO UNKNOWN REASON (S). FACTORS IN THIS ACCIDENT WERE 1) LOW ALTITUDE WHEN THE PARTIAL LOSS OF POWER OCCURRED 2) LOW ROTOR RPM AS A RESULT OF THE LOSS OF POWER, AND 3) THE PILOT NOT BEING ABLE TO TAKE REMEDIAL ACTION DUE TO THE LOW ALTITUDE AND ROTOR RPM.