Forgot to mention multiple flushes and new thermostat. I'm betting on the clutch is bad and can't flow enough air under a load. My wife and son drive in higher RPM ranges and I drive it like I am driving my diesel. I rarely get over 2000 RPM and don't even come close when shifting. They may be out running (heating too quickly) the fan's ability to draw air. A mechanic friend agrees with my theory so that will be the next check. Sometimes it's the simple thing right in front of you that is the problem. Thanks for the suggestions.

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R Todd
98 Ranger
97 F350 7.3 TD
93 Aerostar