There are LOTS of possibilities, but start with a simple test that will tell you a LOT. The fresh/recycle flap is located just behind the glove box. It takes all of 5 minutes to pull out the glove box.
Once you have it out, observe the door behind there. This door selects if you are drawing fresh air or recycled. A/C on a hot day requires this door to work correctly. If the two pink vacuum pods in front of it are shot, then you will never have reliable cold air. Check out this video:
http://www.youtube.com/watch?v=rRP892ytqXg
The door should function like this when you apply vacuum to the green line. It should also do this when you press the recycle button on the console, or when it's hot and you put the A/C on. If the pods are shot, replace them. If the pods hold vacuum but you still don't have the door working correctly with the recycle button, the next likely cause is a bad solder joint on the climate control vacuum solenoid unit. I posted here about that somewhere, with pics.
Once you have verified this door is working as it should, if you still have problems then post back. Check this first, it really is the most likely cause of poor A/C in these cars, and if those vacuum pods have never been replaced then they are almost certainly shot.
Good luck, post back!