The A/C in my 20 year old Volvo has started acting up. Everything is fine up to about 80 degrees ambient, but after that it eventually stops working. Yesterday afternoon it was 80 degrees and the A/C worked for about twenty minutes. It seems as if the compressor stops kicking in. The higher the ambient temperature, the faster this happens. Yesterday I switched it off for a few minutes and then switched it back on. It will give me another five minutes if I do this before it quits again. Somehow I doubt its the charge because it will deliver cold air with ambient temperatures above 90, but I could be wrong. It seems to be control related because the automatic climate control slows the fan down when the compressor isn't running. Any thoughts?