THE PILOT MADE A SOUTHERLY TAKEOFF FROM HIS SHORT/UNEVEN GRASS AIRSTRIP IN HIGH DENSITY ALTITUDE WEATHER CONDITIONS. THE AIRCRAFT WAS LOADED NEAR ITS MAXIMUM GROSS WEIGHT WITH 3 OCCUPANTS ABOARD. ACCORDING TO WITNESSES, IT STALLED AT LOW ALTITUDE. SUBSEQUENTLY, IT IMPACTED THE GROUND IN A STEEP NOSE DOWN ATTITUDE & BURNED. ABOUT 23 MI SOUTH AT LA GRANDE, OR, THE WIND WAS CALM; WITNESSES AT THE ACCIDENT SITE INDICATED THE WIND WAS FROM THE NORTHWEST AT 5 TO 10 KTS. THE DENSITY ALTITUDE WAS ESTIMATED TO BE 4100 FT. THE PILOT HAD FLOWN 2:15 HOURS IN THE PREVIOUS 9 MONTHS.