At a height of about 100 feet after takeoff, the engine faltered briefly. It then returned to full power, but the pilot decided to conduct a low level circuit and land in order to investigate the apparent anomaly. On the downwind leg the engine stopped and the pilot was committed to a wheels-up landing in a paddock. Shortly before touchdown engine power was regained and the propeller contacted the ground under almost full power. The pilot later advised that he had inadvertently selected an almost empty fuel tank prior to takeoff. He had not checked the fuel selector when the first loss of power was noticed, because he had been convinced that he had selected the correct tank.