2006 Volvo S40 Air Conditioning working sporadically.

Last summer, I noticed that the air conditioner in my Volvo began blowing "warm air." I shut the unit off for some time, turned it back on, and it seemed to work fine. However, as the summer progressed, I noticed that it was happening more and more. Oddly enough, the "cooler" it was outside, the longer the unit would blow cold air; however, the warmer it was, the more often it would cut off and blow warm air. It's summer again, and the problem persists. I had a friend check the refrigerant, but it showed to be nominal. I have not been able to determine much else than that. Any idea what can be going on here?