Thanks for the replys.
Some clarification: I've had the truck 10 years and it just started doing this.
It is the fan that stops blowing, don't know if air is diverted or fan stops.
I understand about the clutch dropping out if you are passing or pullin a big hill. That makes sense. Although I have never noticed air temp rise when passing or climbing a hill, but that is not my problem.
Also, my MPG dropped from about 10.5 to 11 to around 8.5 to 9 when this air flow problem arose. Mechanic said he was going to drive it and see if he could duplicate symptoms, he said he hooked a vac pump and guage to lines, and they held a vac. so he feels there are no leaks.
Maybe it has a intermittent leak or only leaks when driving? I don't know about automotive Vaccums and how they affect engine operation or preformance. My expertice is Commercial/ Indust. HVAC.