radiator overflow thing? You mean the resevoir. Those only overflow when the radiator is overfilled and it needs somewhere to runoff. You might want to compression check and see if your radiator is holding pressure. If it isnt you may have either a leak on one of the lines or on the actual radiator itself. It wouldnt be impossible for the radiator to leak at the very top. When the car is cold the radiator wont fill all the way up (resulting in no spilling), but when its hot it should fill the radiator to the top, causing it to leak only then. Dont think zebras when you hear hooves. I'd go to a mechanic and have him do a quick compression check. Takes no more than a minute or two. I'm sure any decent mechanic would check it for free.