The pilot's failure to maintain control of the airplane while operating it at low levels above mountainous terrain and in excess of its maximum allowable gross weight, which resulted in an aerodynamic stall. The pilot's failure to maintain control resulted from an acute cardiac event and incapacitation, hypoxia, or the effects of sedating medications or a combination of these factors. Contributing to the accident was the pilot's improper decision to takeoff above gross weight and without oxygen for the flight in mountainous terrain.