NEW YORK. June 14 ÛÓ The stock market slid, downhill today in moderate trading as investors reacted with less than enthusiasm to President NixonÛªs new economic game plan.</br></br>The Dow Jones average of 30 industrials dropped almost 6 points in the first half-hour of trading, moved up to plus 3 by midmorning, then went steadily downhill the rest of the day. It closed at 902.92, off 12.57 for the clay.</br></br>Declining issues on the -I New York Stock Exchange led advances by 986 to 398 T among 1.726 issues changing hands. Volume totaled 13.2i . million shares compared with</br></br>The broad-based NYSE index of some 1,500 common stocks fell 0.62 to 56.05 and Standard & PoorÛªs 500-stock index was off 0.92 at 106.68.</br></br>The NASDAQ composite index of over-the-counter stocks fell .77 to 103.56, while the Amex price-change index slipped .05 to 22.64. Of the 1.077 Amex issues traded, 285 advanced, and 485 dropped. Volume was 2.12 million shares compared with 2.43 million Wednesday.