Vauxhall Corsa overheating badly and loss of power.

I've been having an ongoing problem with my corsa, it regularly overheats, had waterpump and thermostat replaced. It still persists in overheating. Today I took a 19 mile trip somewhere and i had to make atleast 12 stops to allow the car to cool down. At one stop the car would not even start it was so hot, it was also steaming badly under the bonnet. Now the car ticks over badly. I've been topping coolant up daily. Not one garage can find a problem let alone a leak. I'm utterly confused on what to do. Please help someone.