a/c fan set to Max, at 60/65/70 mph fan cuts off intermittently and goes to defrost/ Floor.
I let up on the gas to 65 mph fan returns to vents/max.
If it was the vacuum more throttle would fix that one would think.
Thank you in advance
Mark/Waco

 

Check the vacuum hose coming out of the passenger side firewall in the engine bay. I had a similar problem on my old truck because the vacuum hose was gone. Not broke, not leaking Gone.

 

More throttle = LESS vacuum. You have it backwards. Manifold vacuum is highest at idle speed and decreases with throttle opening.

Is the fan motor actually cutting out or is it continuing to run but the airflow is being diverted elsewhere, usually the defrost. You need to be precise as to the description of the symptom.

If airflow diverting to the defrost, a vacuum leak, as suggested, is probable as that is the default position of plenum doors.