The airplane made a forced landing following a loss of power from both engines. The pilot reported that he had called the fixed base operator at the airport to have them pull his airplane out and to have 30 gallons of fuel added to each side. He stated that when he arrived at the airport, the fuel had already been added to the airplane. He stated that he performed a pre-flight examination of the airplane which included visual checks of the fuel which was blue in color, consistent with 100LL aviation gasoline. The pilot reported that the subsequent start-up, taxi and pre-takeoff checks were normal and he proceeded to take off. He stated that at about 32 inches of manifold pressure the left engine stumbled and he aborted the takeoff. After taxi back to the runway, the pilot performed another run-up and attempted another takeoff which again resulted in an aborted takeoff. The pilot stated that he then returned to the maintenance area where a mechanic boarded the airplane and a full power run-up was performed. No anomalies were noted during the full power run-up. After the mechanic deplaned, the pilot returned to the runway and was again cleared for takeoff. He stated that he entered the runway, held his brakes, and increased the power to 35 inches of manifold pressure. He stated that he checked the engine gauges and all were normal and he applied full power and proceeded to take off. After liftoff, the pilot noticed a fluctuation in the right engine's manifold pressure and he began a gradual left turn. He stated that the manifold pressure and fuel flow indications for both engines then began to fluctuate. The pilot's attempts to rectify the power fluctuations were unsuccessful and an eventual off-airport forced landing was made. The pilot reported that he had to maneuver the airplane to avoid trees, cars, houses and people during the forced landing. A fuel receipt from the fixed base operator revealed that the airplane had been fueled with aviation jet fuel.