On April 19, 1995, approximately 1330 central daylight time, a Briggs Glasair III, N55EE, was substantially damaged during a forced landing near Crescent, Oklahoma. The private pilot was not injured. Visual meteorological conditions prevailed. The following is based on the pilot/operator report. The pilot was en route from St, Cloud, Minnesota, to Oklahoma City, Oklahoma, and landed at Ponca City, Oklahoma. After refueling, the engine would not start. The battery was recharged, a spark plug was checked for proper firing, and the fuel flow was confirmed by examining the fuel supply line, distributor, and injector. The engine still would not start. After waiting for 30 minutes, the engine was started but it ran rough. Eventually, it smoothed out and the magnetos checked normal. A short test flight revealed the engine would run smoothly at high and low manifold pressure settings and 2,450 RPM, but would run rough at mid-range manifold pressure settings. After landing, the fuel injector was removed and the restrictor valve was replaced. The engine then operated normally. Mechanics suspected a fuel vapor lock. The pilot took off from Ponca City. Near Crescent, Oklahoma, the engine lost power. Suspecting "vapor locking and fuel starvation", the pilot made a forced landing in a wheat field. During the landing roll, the nose wheel hit a small dip in the field and the airplane nosed over. During a post accident inspection by an FAA inspector, no anomalies were found that could account for the loss of power.