Like an open can of soda left out overnight, PepsiCo (PEP -3.18%) stock has lost its fizz lately -- it's down by about 15% over the past year at the time of this writing. Though it has delivered consistent earnings growth, the combination of muted guidance for the year ahead and rising uncertainties about the outlook for the U.S. economy are weighing on the beverages and packaged foods giant.Does the stock's current weakness point to more downside ahead, or are there good reasons for investors to stay optim ...