2002 Ford Taurus Coolant leak

The water from the cooling system is all leaking out. This happens when I drive the car and not when it is not running. The amount of fluid leaking has continued to increase to the point that if I were to drive it I would have to add nearly 2 gallons when I stop driving. The check engine light is on and I had that diagnosed at an auto store. One prob is it needs a tuneup and the other had to do with the crank sendor. Neither of these appear to me to be connected with the leak.
This all began when I was on the freeway and noticed the temp was on hot. I got off immediately at a parts store and let it cool off. Then put 2 gallons of antifreese mix in and continued to drive it. Althought the coolant continued it began slowly leaking and then continued to increase. The temp has never gone up again although it got low in coolant. I kept putting water (not coolant) in and driving until the leak incresed to the 2 gallon point. Now the reservoir has rust residue on it possibly indicating the cap is leaking preasure and water according to the Chilton manual. I am broke until the beginning of next month and hope that a new cap would allow me to drive the car. My question is could it be that that a faulty reservoir cap is the solution to the leaking?