clio overheating?

I have just bought a 1.6 2003 clio . first it started losing waiter through the filler cap but the garage replaced this and also put a new water pump on it because it 'was on its way out' whilst there they replaced all the belt, pulleys and tensioners. i collected the car a 7 miles up the road the car overheated badly steaming through the filler cap. i got it back to the garage and they aren't sure what it can be as its not losing water and doesn't overheat unless its being driven any ideas??????