NEW YORK — A strengthening dollar and worries about an overheated market pounded stocks.

Stock indexes started higher yesterday but turned sharply lower at midmorning as a rebound in the value of the dollar stalled a rally in commodities. Early gains in prices for oil and other commodities had pushed up shares of energy and materials companies.

The sharp swings in currency and commodities markets sent the Dow Jones industrial average whipsawing in a 200-point range, surrendering an early advance for a loss of 104 points. Stocks have fallen in four of the last five days.