air conditions cools for about 15 min then just regular air on 2005 Volvo S40

air condition cool just 15 min then blows just regular air can't find problem, did test compressor fine , blower fine

2 answers
When the A/C is not cooling, are you getting a voltage signal to the compressor clutch? Check this because I've seen these clutches fail when they get hot.
In order for the compressor to work it has to detect refrigerant. If there is any air in the system it would cause this. On the other hand if you turn it off for a while and then turn it back on and it works again you may want to peruse what the other person said about the clutch over heating. Get you system pressure checked first that’s the cheapest. Always start with the simple and cheap fixes.