jb, with everything that's been offered my bet is either 1) Inadequate fan airflow/power/current draw (bad fan selection), not giving you what you may believe to be the "rated air flow"; or 2) Inadequate conductor size providing power to the fan, resulting in reduced current to the fan, reduced airflow and poor performance. In my book you need a top quality two speed fan operating through a relay to provide full power, and a good fan controller to stage the fan operation based on coolant temperature. Just my $0.02, and just to clarify, is it correct that your fan is pulling air through the radiator, not pushing? Pusher fans tend to restrict airflow when they are not needed, increasing coolant temp to the point that they run more than a puller.