I also have a 3.7 Roadster - bought new last year. The fan comes on at low speeds (crawling in traffic) and when the car is stationary but goes off as soon as I get moving. The engine temperature is fine and so I have always assumed this was a feature of the settings on the fan thermostat and that it is not a problem with overheating. It may be worth investigating this before fitting a new radiator.
Ben
