That fan airflow rating is for open air only, with no restriction in front of the fan. Auto radiators are fairly substantial flow restrictors and you can get some moderate pressure drop through them. When that happens, many (most?) of the fans being sold for purported auto use have their airflow just drop into the toilet. So, very likely, as mentioned by others you're not getting anything close to 2400 cfm of airflow, and that is killing the cooling at low speeds.
