[LX5liter]

You don't want to pump the water through the rad too fast, as it won't have sufficient time to cool down. Did you try the stock water pump with the aluminum rad? Maybe the Milodon (reverse rotation one, I assume) is cycling the fluid through too fast. What mixture of water/coolant are you running? Are you losing any coolant at all?

edit: sorry to repeat PKRWUD, he must have posted just before me.