A/C problem on 2005 Volvo S40

I have a 2005 Volvo S40 with 64,000 miles and live in Central Florida. The AC has worked fine until about a month ago. The problem is that the AC intermittently stops blowing cold air. The fan stays on and the compressor seems to be turning on, but I’m basically getting warm air out of the vents. Some days it does it a lot, other days it works just fine.