1999 GMC Sierra Cold air goes to hot

Truck a/c system will blow cold air sometimes usually for about 20 mins. Then will start blowing hot if you cut the a/c off for about 20 mins. It willl blow cold again mechanic before me supposedly changed an orfice tube filter and the compressor I have had the system down when I replaced the heater core in the dash and had to break the evaporator out vaccumed it down and recharged to correct charge