My 2002 Subaru Forester has intermittent A/C issues - blows hot air sometimes...

What is your question?

When I run the A/C, after a short time, warm air blows out of the vents. I had the freon charged / refilled and the system checked for leaks by mechanic but no leaks were found. It had been in 90's when it first started but when I picked it up from mechanic it was cooler, only 83 degrees outside. I did not notice it happening then but a day later, it still doing the same thing again. Any suggestions greatly appreciated!