1989 Acura Legend Air Conditioner stops cooling after 30 mi

Air Conditioning problem
1989 Acura Legend 6 cyl Two Wheel Drive Automatic 250k miles

Hello, I have an '89 Acura Legend which has well over 200,000 miles on it. Basically the I use the car to do courier work, so naturally it's driven quite alot. I stay on top of all maintenance work that needs done in order to extend the life of the car as long as possible. In all honesty I have a good mechanic and he really takes good care of me (actually my car). He has charged the A/C, checked for leaks, and yet I still can only get it to cool for about thirty minutes before it stops cooling the car. The A\C system is charged, and upon inspection appears fine, but after driving for 30 minutes or so it slowly peters out. Has anyone else ever experienced such an issue before, or are familiar with what's causing it to slowly stop cooling after it's been on for a short while? I'd certainly appreciate some insight regarding this problem if anybody has a good idea what might be happening, as it's hot & humid now and it just makes these deliveries I do anything but comfortable! Thanks in advanced, & I do appreciate anyone taking time to read my question!