Stock prices drifted lower today in selling apparently prompted by impatience over the market's recent sluggish performance. The Dow Jones average of 30 industrials dropped 5.15 to 2,085.35 by noontime on Wall Street. Losers outnumbered gainers by about 3 to 2 in nationwide trading of New York Stock Exchange-listed issues, with 488 up, 731 down and 564 unchanged. Analysts said the market's sluggish recent behavior apparently convinced some investors that stocks were unable to break out of the trading range in which they have fluctuated for several months now. They also noted an absence of conviction about the business outlook to encourage any buying. Some prominent analysts and policymakers, including Chairman Alan Greenspan of the Federal Reserve Board, have been warning of the dangers of revived inflation. Yet the price of gold, a primary gauge of inflationary expectations, has fallen below $400 an ounce this week for the first time in more than a year and a half. Similarly, brokers say, there is widespread confusion and uncertainty among investors about prospects for continued economic growth in 1989 under a new administration whose makeup and policies are not yet known. Among actively traded blue chips, International Business Machines rose \ to 113{; American Express was unchanged at 27~, and American Telephone & Telegraph slipped [ to 26]. Reebok International, which reported a higher order backlog than a year ago, rose ] to 11 in active trading. The NYSE's composite index of all its listed common stocks dropped .17 to 152.47. At the American Stock Exchange, the market value index was down .63 at 298.34. Volume on the Big Board came to 76.28 million shares at noontime, up from 57.29 million Wednesday, when activity was diminished somewhat by the observance of the Jewish holiday Yom Kippur.