Aug. 22 (Bloomberg) — The Standard & Poor's 500 Index fell from an all-time high as investors awaited a speech by Federal Reserve Chair Janet Yellen for clues on whether interest rates may rise sooner than anticipated.

Aeropostale Inc. sank 8.4 percent after forecasting a wider-than-estimated loss. Dynegy Inc. climbed 12 percent after agreeing to buy $6.25 billion of power plants from Duke Energy Corp. and Energy Capital Partners. Gap Inc. rose 4.2 percent after the retailer reported quarterly profit that topped analyst estimates. Ross Stores Inc. increased 6.9 percent after raising its annual forecast.

The S&P 500 fell 0.2 percent to 1,988.44 at 9:48 a.m. in New York. The benchmark gauge is heading for a 1.7 percent weekly increase. The Dow Jones Industrial Average lost 21.35 points, or 0.1 percent, to 17,018.14. Trading in S&P 500 companies was 16 percent below the 30-day average for this time of the day.