I'm guessing bad radiator if the stock radiator kept it cool, and this one doesn't. While at idle, place a couple of dryer sheets on the front of the grille. If they won't stay, air flow is the issue. If the air flow is good, cool the car down, remove the lower radiator hose at the radiator, hand over the outlet, have someone fill the radiator, leave the cap off and remove your hand from the outlet. If the water gurgles out, it's a bad radiator. A strong flow and then it's something else. But that way you take the new radiator out of the equation. Good luck. Cooling issues are never easy.