Why does my A/C not blow as hard after time has passed during a long trip?

What is your question?

2003 Dodge Ram 2500 5.9 H.O. I get in the truck and the A/C blows cold with great air volume but after about an hour, it still blows cold but with a lot less volume.

What makes this problem better or worse?

Turning the truck off and then turning it back on 20 minutes later and its back to normal

How long has your 2003 Dodge Ram 2500 had this problem?

Month