I have had the same a/c issue's as several others have had with the a/c only working at higher rpms or when there are cooler temps outside. The a/c clutch seems to be the solve for me. I used a c-clamp to lock the clutch up to the pully to remove the center bolt and then a screwdriver to gently pry the clutch out enough pull the clutch off the center shaft. there was a small washer between the clutch core and coil that I removed to get the gap smaller. before doing the above steps I waited till the compressor was not engaged and blowing warm air from my vents, then I lightly tapped the clutch in and it snapped right to the coil. This was my indicator that the clutch was wore out. I confirmed it by removing the washer and getting the clutch closer to the coil. New clutch hub at Oriellys is $35. 10 minute fix.
Joe