UTAH'S DECEMBER INDEX OF LEADING ECONOMIC INDICATORS SLIPPED .5 PERCENT FROM NOVEMBER, OFFSETTING PART OF THE PREVIOUS MONTH'S 1.3 PERCENT INCREASE. FIRST SECURITY BANK OF UTAH, WHICH PRODUCES THE LOCAL INDEX, SAID THE INDEX HAS INCREASED TWICE AND DECLINED THREE TIMES IN THE PAST FIVE MONTHS. FIRST SECURITY ECONOMIST KELLY K. MATTHEWS SAID THE UTAH INDEX'S GROWTH MOMENTUM FROM DECEMBER 1987 TO DECEMBER 1988 WAS 3 PERCENT, MAINTAINING 1988'S TREND OF 12-MONTH GROWTH RATES AT 3 PERCENT OR HIGHER. INDICATORS USED IN THE INDEX INCLUDE NEW AUTOMOBILE LOANS, CONSUMER LOAN DELINQUENCY RATES, UNEMPLOYMENT CLAIMS AND NONRESIDENTIAL CONSTRUCTION PERMITS. ALSO TAKEN INTO ACCOUNT ARE NEW DWELLING UNIT PERMITS, MANUFACTURING WORKWEEK HOURS, NONAGRICULTURAL JOB PLACEMENT, NEW AUTOMOBILE LOANS AND NEW CORPORATE FILINGS.