AC blowsGreat while stopped or low speed’s on fresh or revert. After you sustain a speed for a short period of time the blower motor is still turning but it is not allowing the air to come out any of the vents. Any suggestions.
Have replaced the vacuum pump and changed out or cracked vacuum lines that I can find

 

Have you checked the refrigerate (R13) pressures? Is the compressor clutch engaging properly? You may need to pull a shim out of the clutch, if you pull the clutch off make sure you add a drop of blue loctite to the bolt that holds the clutch on.

 

If freon is low it's possible that the evaporator is freezing and icing up blocking air flow. The more it gets blocked the more it freezes until it becomes a solid block of ice. Does it start to get more airflow if you turn off ac and let fan run in high? The longer you leave it that way the more it will defrost until the evaporator is cleared and allowing full air flow. I had a Chevy truck that did that at the first of each year and I would add 6 oz of freon to stop it.

 

When the A/C drops off, try to change the settings on the controls. It sounds like the doors inside may be closing. When you slow down, does the A/C return to the vents?