Witnesses observed the airplane on a tight downwind for landing. The airspeed seemed slower than other airplanes in the traffic pattern. The nose of the airplane was observed to pitch up and down while on the base leg. The airplane over shot the turn to final and started a turn to the left - estimated between 35 to 45 degrees. The left wing dropped down 90 degrees as if in a stall, the nose pitched down, and the airplane collided with the terrain.