On May 11, 2005, about 1915 eastern daylight time, a homebuilt RV-6A, N42RZ, experienced a loss of engine power while in cruise flight, and was substantially damaged during a forced landing near Kennett Square, Pennsylvania. The certificated private pilot was not injured. Visual meteorological conditions prevailed and no flight plan was filed for the personal flight conducted under 14 CFR Part 91. The pilot reported that on the day prior to the accident, he experienced difficulty restarting the engine and suspected a vapor lock problem. He installed a blast tube to aid the cooling of the engine fuel pump and conducted a test flight the following day. During the test flight, while the airplane was at 2,000 feet, it experienced a loss of engine power, after the pilot cycled the auxiliary fuel pump switch. The pilot turned toward a nearby airport, switched fuel tanks, placed the mixture control to full rich, and confirmed that the fuel pump was in the "on" position. The airplane continued to descend and the pilot performed a forced landing to a field. During the landing, the airplane struck a ditch, and flipped over. Examination of the engine by a Federal Aviation Administration inspector did not reveal pre-impact malfunctions. The airplane was equipped with a facet cube-style electric auxiliary fuel pump. The pilot reported that subsequent troubleshooting of the fuel pump revealed that it operated intermittently and restricted fuel flow to the carburetor.