THE PLT TOOK OFF WITH AN 800 LB SLING LOAD & USED 100% TORQUE TO EXPEDITE CLEARING POWER LINES. WHEN CLEAR OF THE LINES, HE STARTED A CLIMBING TURN & REDUCED THE TORQUE TO ABOUT 80%. AT APRX 300 FT AGL & 50 TO 60 KTS, THE ENG BEGAN SURGING WHICH CAUSED THE HELICOPTER TO YAW REPEATEDLY. SUBSEQUENTLY, THE ENG LOST POWER & THE PLT RELEASED HIS LOAD & MADE A FORCED LANDING IN A WOODED AREA. HE USED THE COLLECTIVE TO EXTEND HIS FLT TO AN OPEN AREA. HE REACHED THE INTENDED TOUCHDOWN SPOT, BUT 'RAN OUT OF PITCH & MADE A HARD LANDING.' THE HELICOPTER ROLLED TO THE RIGHT & THE MAIN ROTOR BLADES STRUCK THE GROUND. AN EXAM OF THE HELICOPTER & ENG REVEALED NO REASON FOR THE LOSS OF POWER. THE DENSITY ALT OF THE CRASH SITE WAS COMPUTED TO BE 11,093 FT. THE HELICOPTER HAD AN ALLISON 250-C30P ENG WHICH HAD RECENTLY BEEN INSTALLED PER STC SH296NM. THE FLT MANUAL WARNED THAT WITH THIS ENG INSTALLED, BOTH FUEL BOOST PUMPS SHALL BE ON DURING ENG OPERATION. THE ENG OPERATED NORMALLY DURING A FUNCTIONAL CHECK.