BEFORE FLYING A SHORT DISTANCE TO THOMASVILLE TO REFUEL, THE PLT ADDED 3 TO 4 GAL OF AUTO FUEL TO THE ACFT. AT THOMASVILLE, THE ACFT WAS SERVICED WITH 20.1 GAL OF AVIATION FUEL. WHILE TAXIING TO TAKEOFF, THE ENG STOPPED & WAS RESTARTED TWICE. AFTER DEPARTING THOMASVILLE, THE PLT FLEW TO THE VICINITY OF A FRIEND'S FARM ABOUT 11.5 MI AWAY. WHILE MANEUVERING IN THAT AREA AT ABOUT 300 FT AGL, THE ENG SPUTTERED, STOPPED & RESTARTED SEVERAL TIMES. A WITNESS REPORTED THAT THE ACFT WAS GRADUALLY LOSING ALT & SPEED WHEN IT SUDDENLY MADE A 'NOSE DIVE INTO THE GOUND.' ABOUT 20 SEC AFTER IMPACTING, A FIRE ERUPTED. THE PLT WAS FOUND ABOUT 30 FT FROM THE WRECKAGE SUFFERING FROM INJURIES & BURNS. HE WAS HOSPITALIZED, BUT DIED 23 DAYS LATER. THE PASSENGER DIED ON IMPACT & THE ACFT WAS DEMOLISHED BY FIRE. NO PREIMPACT PART FAILURE OR MALFUNCTION WAS FOUND DURING THE INVESTIGATION.