Water in gas tank...more common than you might suspect...Do the symptoms get a little better after driving for a while?(the water will eventually mix with the fuel after continued driving, and the symptoms might improve..until you park and let the water settle out).

Water sinks to the bottom of the tank below the stand pipe..however if enough gets in it can be right at the level of the stand pipe, and so everytime you come to a stop, it sloshes into the fuel intake and causes rough idle or a stall and occasional rough starting...adding a little isopropylene might help..but typically if there is a lot of water, it will require constant use of this for a while..another solution is syphoning the bottom of the tank...may need a weight on the hose to get it to sit on the bottom. Have had this happen.