Volvo Repair: Climate Control, proper diagnosis, volvo 850

Question
After running the A/C for 15 minutes or so, my '96 Volvo 850 stops chilling the air even though the compressor is still running. When I turn the A/C off and back on it will work only work about half the time. I had it in the shop a year ago for the same problem and they said it needed freon, which seemed to solve the problem last summer. It did it again this summer and we took it into a quickie lube place and they said they couldn't find a leak and the pressure was correct, but there were contaminants in the system and their $130 evacuation and refill would fix the problem. It worked for a month and now we're back to the original problem. Any thoughts or help?

Answer
without being able to see it I really can not give a proper diagnosis.sounds like it is loosing freon and is now low again.there may be a leak at the evaporator which you may not be able to see.I would have tracer dye added to the system so if there is a leak in the future it can be checked with an ultraviolet light.