The pilot reported that the cross-country flight proceeded normally until approximately 25 miles from the destination. At that time, the engine speed decreased unexpectedly. His attempts to increase engine power by pushing in the throttle had no effect. The pilot noted that he switched the fuel selector to the right tank and then to the left tank with no effect. He returned the fuel selector to the “both” setting and the engine regained power. He elected to immediately divert to the nearest airport. However, the engine lost power again and he ultimately executed a forced landing to a corn field. The airplane nosed over and was substantially damaged during the emergency landing. A post accident inspection did not reveal any anomalies associated with a pre-impact engine failure or malfunction. No evidence of a fuel spill was observed at the accident site, and no fuel was observed in the fuel tanks after the accident.