Mysterious Coolant Leak

I have a 2000 Ford Taurus, 146k. It's my wifes car, PLEASE HELP. The water pump gave out, so I had it replaced. Then the radiator went out, replaced. I flushed the system, six times until clear water showed, I did this a couple of times isolating the heater core as well. Replaced the coolant reservoir. All is well, until I go uphill. Water escapes through the reservoir. Had pressure test and dye test done, all is well. Mechanic told me there are no air bubbles, oh new thermostat as well. Now my wife will not go down the hill to go grocery shopping. She says it's up to me! HELP!