2000 Ford Taurus coolant overflow

My ford taurus has been running great. The problem began when I came home, ran my groceries inside and returned to my car to notice a huge puddle of coolant on the ground. After a thorough check I could not identify were the leak was coming from. I took it to a mechanic and I had work done to repair it (new water pump, thermostat, repair head and replace head gaskets). The problem is still occuring. What is happening is I can be driving around, my car does not overheat, and then as soon as I stop and turn off the car the coolant is actually expelling from the coolant jug. You can see the coolant puddleing around the jug lid. I was told my the mechanic that the system must be overpressurizing but with the repairs I did he believed it should have fixed the problem. I have come to a stand still and no reputable dealer can give me an answer.