The Dow Jones industrial average set its seventh straight record high, edging up 1.08 points to close at $5,601.23, and raising fears that the market may be overextending itself. The NYSE composite index slipped 0.41, ending the day at 351.29. Trading was heavy on the New York Stock Exchange, and declining issues outnumbered advancers slightly. After a dive early in the day, the Nasdaq recovered some and closed at 1,087.22, down 8.16 points. The Standard & Poor's 500-stock index fell 0.93 to 660.52, and the American Stock Exchange's market value index fell 1.84 to 560.62. Gold in London finished at $403.45, off $1.80.