Air condition problems with 07 gmc acadia

[cartalk1504]

have an 07 gmc acadia with 50,000 miles…on trips of 45 minutes or more the ac starts blowing just vent air and barely enough to move a tissue if held in front of it… sometimes it sounds like it is blowing really hard under the dash but not coming out of the vents. Sometimes you can cut it off and crank it again and it will be fine but most of the time it takes atleast 30 minutes of just cutting it off to get it fixed…