It sounds very much like your Hayden fan is not up to the job. The claimed CFM ratings are fairly meaningless. I assume the rad was adequate with the engine-driven fan, so i doubt that it is the problem. The shrouds that come with aftermarket fans are not nearly as efficient as the full-coverage factory shrouds, so the aftermarket fan has to be even more powerful to compensate.

The hardest pulling aftermarket fans are made by Spal: http://www.spal-usa.com/html/dampframe.htm. They are not cheap, but they get the job done. I went through many fans until I got a Spal that was up to cooling my V8.

There are factory fans from other cars that are quite powerful and sometimes have shrouds that can be adapted to fit a Volvo rad. I had to go with a slim-fit Spal because of extreme space restrictions of the V8, but you can get the job done much more cheaply with parts from a junkyard. Go here for the plan: http://www.linkline.com/personal/dbarton/fanconversion/.

Good luck!
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'84 "285" rice-eater with lotsa hp & performance goodies