1999 AC question

Ok guys got one for ya. I have a 1999 3/4 4x4 454 and I live in AZ it gets hot. It has cold ac when when its like 90-98 degrees out then when it gets a little hotter like mid day it just does not seem to cool. I have put a vaccume on it for 1 hour. Then charged it up. When I checked static pressures with temp it was close with in 3 degrees. Why the change in cooling? I have a 96 chevy truck works fine in the same temp out here in AZ. Any help would be great thanks Jim