When I am accelerating or going up a hill or pulling a heavy load my air conditioner quits blowing through the vents when i am on flat ground or going down hill or holding a steady speed the air does fine any help would be appriciated also i have a 98 f1505.4L 4x4

 

if it goes to the defrost position then you have a vacuum leak

 

the problem is low a/c compressor. try check it out.

 

A/C system has a vacuum leak. Check the hose that goes thru the bulkhead for chaffing, around the battery area for evidence of battery acid attacking the plastic vacuum lines or the vacuum tank in the right front fender has a leak. You may also have a vacuum reservoir behind the battery.