The United States government said Tuesdaythat its main forecasting gauge of future economic activity edged down 0.1 percent in September, its second decline in the past three months. The Commerce Department's Index of Leading Indicators rose 0.5 percent in August and 1.5 percent in June but fell 0.7 percent in July. Economists believe this rollercoaster performance is a signal that the economy, which was expanding at a robust pace in the first six months of 1988, is slowing down.