What do you suggest?

First, the radiator was replaced, are you sure the system was filled properly? If I recall, there is a proper procedure for filling and bleeding the system If I get you, you start the engine, and the coolant in your reservoir starts bubbling instantly. That is a major sign of head gasket failure. Combustion gases are escaping past the gasket and essentially blowing bubbles in your coolant. Check the oil dipstick and check the condition of the oil. If coolant and oil have mixed, you will see a greenish mayonnaise or milkshake on the dipstick. It will not look like normal oil. In this case I would definitely do a cooling system pressure test. This will pressurize your cooling system and you can watch for a pressure drop. If it is leaking, you will see a pressure drop. If there is no external leak, the water may be going into the cylinders.

I had the same problem with my 03 Sebring. We changed the heater housing on top of the reservoir and then found that the reservoir itself was the problem. The nipple leading off was cracked. Changed the reservoir and it's no longer boiling.