My car is overheating in traffic with or without A/C on. but the odd part is that it does not do it every time we are stuck in traffic. I replaced the thermostat and the fan seems to be working properly. How can I tell if there is a blockage? I am in Colombia south America so things are a little different than what I am used to. For one the radiator cap is not like the ones I have seen in the US. Any thoughts or suggestions would be greatly appreciated. thanks