On April 6, 1996, at 1820 Alaska standard time, a skid equipped McDonnell Douglas, H-369 helicopter, N58268, registered to and operated by Tundra Copters, Inc., of Fairbanks, Alaska, experienced an unscheduled engine spool down during cruise flight and was forced to land in trees. The positioning flight, operating under 14 CFR Part 91, departed the Trophy Lodge at Big Delta, Alaska, and the destination was a field site located 30 miles east of Big Delta. A company visual flight rules flight plan was in effect and visual meteorological conditions prevailed. The certificated commercial pilot, the sole occupant, received minor injuries and the helicopter was substantially damaged. During a telephone conversation with the pilot on April 8, 1996, he stated that during cruise flight the engine spooled down. He heard the low RPM warning audio but the engine relight activation light did not illuminate. He entered autorotation but did not roll the throttle to the off position. During the autorotation the engine "spooled back up." The pilot applied collective and started to climb out. The engine again spooled down and the pilot attempted to complete the autorotative landing. The pilot stated that during the flare the engine spooled up again and he landed hard with forward velocity. The helicopter nosed over and remained inverted. The engine was still running after the helicopter stopped and the pilot had to shut the engine down. The airframe was examined and the fuel system was pressure checked and found to maintain 8 inches of Hg for 2 minutes. The engine fuel system was pressure checked and found to maintain 8 inches of Hg for 2 minutes. Fuel samples looked clear with no visible debris. The engine was sent to Allison for a test run. Results showed that the engine started normally. Acceleration and deceleration tests were normal. Power output for the engine was rated at 4.9 percent below normal. The airframe fuel cell contained fuel.