My bf changed the thermostat on my 95 mazda protege around a month ago and it hasnt overheated that is untill today...however it seems to only overheat when im stopping like at a red light but then it starts to go back to normal again as im driving. I pulled over and put some coolant in but saw that it started leaking out under the hood in the front on where the passenger side is. Could it be the thermostat again or the radiator? If the radiator, is that expensive to repair?