Poor functioning A/C in 2004 GMC Sierra 1500

I have paid for a dealership to replace my A/C compresor, condensor, and filters, which they diagnosed as being the problem with my a/c system. The original problem was that while sitting idle one day, hot air started coming through the vents instead of cold. When I would shut entire vehicle down and restart it, would blow cold again. Now they have replaced three compressors and it is still not right. When sitting idle, set at 60 degree temp, only blows barely cool air. When I checked the PSI, reads 65+. I have suggested they check the sensor and/or dash controls, but they say that isn't the problem. Any ideas?