Slowly poor water over the engine and it starts right up

We'll go to the mall, have diinner, come out and the car won't start. After a long wait it starts. Now we've got to the point where we keep an extra gallon of water in the trunk and when it doesn't start, we open the hood and slowly poor water over the engine and it starts right up. Mechanic is unable to duplicate the problem so they say they can't help us. Anybody have a suggestion? There's a sensor, solenoid, relay or something that shuts down the starter.