NEW YORK (AP) _ The stock market, famed for zigzag changes of mood, has pulled off another big surprise in the past year and a half. It has suddenly turned flat.

All last year, the daily closing reading of the Dow Jones industrial average never ventured as much as 10 percent from its low to its high.

It repeated that less-than-10 percent pattern in the first half of 1993. Over the last 18 months, the difference from the Dow average's closing low of 3,136.58 (Oct. 9, 1992) to its closing high of 3,554.83 (May 27, 1993) has been just 13.33 percent.

That represents one of the narrowest ranges over such a lengthy stretch in modern market history.

''Bulls are frustrated; bears are frustrated,'' says David Shulman, an analyst at the investment firm of Salomon Brothers Inc., in a commentary titled ''Stalemate.''

''The stock market is not going up, and the stock market is not going down.''

Indeed, this year the Dow average, with its relatively narrow sample of big-name stocks, has overstated the strength of the overall market somewhat. While it rose 6.51 percent from New Year's through June 30, the broader Standard & Poor's 500-stock index, New York Stock Exchange composite index and Nasdaq composite index each settled for gains of between 3 percent and 4 percent.

Yet as investors in International Business Machines, Philip Morris and a wide range of other companies will readily attest, an absence of volatility in the overall market hasn't kept many individual stocks from taking dramatic swings.

From mid-1992 to mid-1993, IBM fell almost 50 percent and Philip Morris almost 35 percent.

Investors' touchy temperament was on full display in last Wednesday's market activity. While the market averages were posting small and mixed changes, no less than 10 stocks - three on the NYSE and seven on Nasdaq - fell more than 15 percent apiece.

''Recently, it seems that when investors lose confidence in a company, the stock doesn't just decline, it plummets,'' says George Putnam III in his Turnaround Letter advisory, based in Boston.

The standoff in the market averages is widely attributed to the mutually offsetting effects of two strong forces - heavy demand for stocks created by a flood of money into mutual funds, and a corresponding outpouring of stock offerings from companies going public or adding to their equity base.

''One of the fundamental differences between the 1980s and the 1990s cannot be overemphasized,'' says Shulman. ''In the 1980s corporations were net buyers of common stocks. In the 1990s corporations have reverted to their natural role of net sellers.

''Implicitly, corporate managements now believe that stocks are expensive, in sharp contrast to the 1980s when they acted on the belief that stocks were cheap.''

Mutual fund managers, struggling to manage an influx of money, also worry that stocks are no bargains, but they are limited in what they can do about that situation.

Putnam suggests that that would explain why individual stocks are subject to dizzying sell-offs, while the market as a whole barely budges.

''Professional investors are nervous that the stock market is too high by many historical measures, but they can't find any place other than stocks to put their money,'' he suggests.

''Hence, either they skittishly jump from stock to stock or sector to sector, or they park their money in the big 'safe' Dow stocks.''