1995 Toyota Corolla Station Wagon

Your help is really appreciated. My 1995 Tioyota Corolla station wagon has recently started blowing hot air from my air conditoner. The problem is sometimes it blows cold and all of a sudden it will start blowing hot air. I do not have to do anything. It does not seem to matter if the car is warm or cold. I took it to a mechanic and they added freon and the dye. The mechanic said it has no leaks and as soon as I bring it to the mechanic it blows cold air so they cannot tell what is happening. I have an elderly parent that cannot stand this phoenix, az heat and I must find out the problem so I may take her to her doctor visits. Please advise if you have any ideas. Thanks
Shirley McClure
Phoenix, Az. :Roll: