air conditioning doesnt cool consistently

I have a 93 ford aerostar and im trying to get the ac right it blows real cold sometimes and others pretty warm accelerating and deaccelerating effect it some but it doesnt seem to be situational. the charge is at factory specs and the low side switch is good i was told it may be the orifice tube. i purchased one but im not sure where it goes or if its the problem. my question is do you think thats the problem and how do i swap it out and if you think it isnt what do you think the problem could be