On February 24, 2004, about 1030 eastern standard time, a Cessna 152, N4669L, was substantially damaged during a forced landing, after a loss of engine power near Spencer, Massachusetts. The certificated flight instructor (CFI) and a passenger were not injured. Visual meteorological conditions prevailed and no flight plan had been filed for the instructional flight conducted under 14 CFR Part 91. The CFI stated he was conducting an introductory flight for the passenger. They departed Worcester Regional Airport, Worcester, Massachusetts, about 1015, and flew over the passenger's house. The airplane was in a climb at 2,800 feet, when the engine lost power. The pilot utilized the emergency procedures checklist, and the engine regained power; however, the engine's performance began to fluctuate from a partial power to a no power condition. The CFI attempted to land at the Spencer airport; however, he was not able to maintain altitude and performed a forced landing to a snow covered field. During the landing roll, the airplane nosed over. The airplane had been operated for about 98 hours since it's most recent annual inspection, which was performed on December 3, 2003. The pilot reported 822 hours of total flight experience, which included 220 hours in the same make and model as the accident airplane. Examination of the airplane and engine by a Federal Aviation Administration inspector did not reveal any pre-impact mechanical malfunctions. Compression was attained on all cylinders. Fuel was observed in both the left and right fuel tanks, and a fuel sample did not reveal any contamination.