The government's chief economic forecasting gauge rose a healthy 0.6 percent in December after having fallen during the previous month, the Commerce Department said Wednesday. The December jump in the Index of Leading Economic Indicators, which is designed to predict future economic activity, followed a 0.2 percent decrease in November and a 0.5 percent rise in October. The index has alternated between dips and gains for the past nine months, which many economists believe is a signal the economy is headed for a slowdown in growth that would help keep inflation in check. The economy grew a robust 3.8 percent in 1988, but the slowest quarterly performance in two years was recorded during the October-December period.