2004 GMC Sierra Lack of A/C cooling on warmer days

About two weeks ago I had an incident in which I was idling and my a/c started blowing hot air. I shut it off and restarted it, at which time it blew cold air again. After doing this again a few days later, I took it to the dealership. They diagnosed it as being a bad compressor and condensor and changed the parts out, as well as filters. When I picked it up, it wasn't blowing cold air, only semi-cool. I left it and three new GM compressors later, they told me it was fixed. I went and picked up in low 80's temperature outside and it blew nice and cold air. Yesterday it was in the low to mid 90's and it blew only mid-cool air again. I checked the PSI and it read 65-70, indicating way over the max charge level. This morning when it was only about 70 degrees out, I checked my a/c with the meter again and it read about 45 PSI, at the top end of "full charge." I am now thinking it had something to do witih the ambient temp outside as to whether my a/c functions properly. Any suggestions as to what it could be? The dealership said they want to make it right, but after more than one week to begin with, I am feeling uncertain. A mechanic friend suggested it might be the fan clutch?