Air conditioning only runs when car is going a constant speed

What is your question?

Its on high nothing comes out unless your going a constant speed I.e. highway. You can't change the speed of air flow or it will shut off even though its on

What makes this problem better or worse?

Better if you are moving a constant speed and worse if you shut it off

How long has your 2006 Dodge Caravan had this problem?

Probably almost a year