It sounds like you may have an air pocket. Correct me if I'm wrong but when you replace the radiator you are supposed to fill with coolant and run it with the cap off until the engine heats up enough and the electric fan turns on (do not drive just let it sit and run) This allows it to circulate coolant and it will remove the air pockets. Also, caps are pretty cheap and I don't think it would hurt to get a new cap. Even if it doesn't leak the cap still plays a big role- it pressurizes the system and therefore changes the boiling point of the antifreeze/water. So, get a new cap - when the engine is cool remove the cap (not when hot) and make sure there is enough fluid in the radiator - run without cap on till fan comes on - put cap on and fill overflow tank. Should take care of the problem.