91 Nissan 240 SX

I have ahad this car in the shop for 2 months attmepting to get the air going.At first it blew cool but not cold. The shop that has the car has replaced several different parts includeing installing a used compressor at a charge of $280 total.It still does not work and the shop tells me that it is the compressor. 8 different used compressors later it is still doeing the same thing. Out of the top vents the air will blow cold at an idle when but at higher RPM it does not blow as cold.If you keep the adjuster of 1-4 low to high on about 1 to 2 it will blow cooler than 3-4.One last thing. If you put the vents on bylevel it only blows hot air on the bottom vents. The shop wants me to shell outabout $500 on a new compressor but common 8 compressors yea they were used but all haveing the same problem. Any ideas would be great. They also say that all the doors are opening and closeing as should. So they do not understand why the bottom blows hot air