I just started having a problem with the air conditioning in my '96 960. It works fine around town. When I get on the highway, within a few minutes it no longer blows cold air, only warm. I pulled over and the compressor wasn't engaging. Within a few minutes of being parked, the compressor came on, and I got cold air. Back up to highway speeds, no more cold air. When I got off the highway, within a few minutes the compressor came on again, and I was fine the rest of the way home. What could be the cause? Do I need to add refrigerant, or could it be the pressure switch? I've already had to replace the pressure switch two years ago. At that time I had no A/C at all. The evaporator was replaced at the same time. Any advice is appreciated.