Stocks plunged in the final minutes of trading Thursday, sending down the Dow Jones industrials 678 points, or more than 7 percent, to their lowest level in five years after major credit ratings agency said it was Considering cutting its rating on General Motors Corp. According to preliminary calculations, the Dow fell 7.3 percent, to 8,579.19. Broader stock indicators also tumbled. The Standard & Poor's 500 index dropped 7.6 percent, to 909.92, while the NASDAQ composite index fell 95.21, or 5.47 percent, to 1,645.12.