Stocks fell on Monday, reversing strong gains from the previous week, as investors considered recent mergers and acquisitions. Economic readings are convincing investors that the economic recovery is gaining ground, sending U.S. stocks soaring to their highest levels in four months Friday. Stocks are expected to end September with the biggest monthly gains in over a year. But investors worry that the health of global financial institutions remains a concern, especially with the U.S. government's seizure of three credit unions on Friday. High unemployment, low home prices, and the government's large budget deficit are other issues of concern to investors. In company news, Southwest Airlines announced a $1.4 billion offer for AirTran Holdings (AAI), the parent company of AirTran Airways. The bid amounts to a 69 percent premium above AirTran's Friday closing stock price. The announcement sent AirTran stock up 61 percent. Shares of Southwest rose 10 percent. --MORE