MY 99' dodge 1500's a/c works great--nice n cold below around 90 degrees, but over that it works on and off. Mostly off the hotter it gets.

I had it checked out they , the shop, told me it was over charged and they fixed that to correct charge levels. It worked super until I shut it off and restarted the truck.

The a/c vents change with vacumn pressure..up hills it changes to windshield defrost mode, and if I let up on the gas just a tiny it, it returns the air to the correct position.

Any theories....

 

Sounds like a vacuum line was knocked loose when the shop did the work. There's a DIY or thread around here dealing with "Wild Vents".

 

+1, also if you have cruise control, check the hose under the battery tray.