Minivan having some A/C issues - help? (vehicle, auto, buying)

[AlaMommy]

We have a used minivan and this is our first summer with it. One day a few weeks ago I decided that the A/C could probably work a little better. The fan isn't exactly strong, but it seemed as though it could be colder. We ended up buying some freon and someone from the auto store was nice enough to put it in the vehicle for us, seeing as my husband or I had never done it before. She mentioned that the generator was kicking on and off and shouldn't be.
So it seemed to be colder/better for a bit, but not long. We went on a long road trip (we're in the southwest and sun was beating down), and it was hot in the car. The A/C was not helping at all. I did notice that if I wiggled the hot/cold selector knob that cold air would blow for a couple minutes.
After returning from the trip, we took the vehicle in to get the A/C serviced. They added freon, changed a filter? etc. to a tune of 80 bucks. All better it seemed!
It's been about 2 weeks since the service and we just noticed that it wasn't blowing cold air. I'd say it was 'room temperature'. I wiggled the knob that chooses floor vent, max a/c, dash, etc. and the air actually seemed to blow hot! Then I wiggled the hot/cold knob and it started blowing cold air finally. We drove around a little longer to see if it would keep up, seeing as on our road trip it only made it last a minute or two, and it was still blowing cold air 10 minutes later. And for the record, we've run it on Max A/C the whole time, I wasn't trying to get cold air blowing in any other setting.
The next time I go out in the car, I'm going to make sure it's blowing cold air to start, whether I have to wiggle a knob or something, and see how it runs while I'm out.
Why are we having such a problem with our A/C? I can't imagine there is a leak, or they would have fixed it when we serviced it, right?
Is it possible that something is wrong with the knob?
This is a frustrating problem being in the south, where A/C is quite important (and two young kids in the car).

[Drover]

You have a leak in the system. It's a high-pressure system so even a pinhole leak is enough for the coolant to escape from. You'll need to have a shop that knows what it's doing find the leak and fix it. Don't use any more store-bought A/C recharger kits; they have leak sealers in them that can plug up lines and muck up the compressor and/or condenser if used too much, making a bad situation far worse.

[sunsprit]

you need to take the van in to a shop that can properly diagnose the control system as well as the mechanical refrigeration system.

there may ... or may not ... be a leak in the refrigerant system, but you mention that you were able to get cold refrigerated air at one time just by jiggling the system controls.

This is a lot more technical work than just guessing that the refrigerant is low and continuously topping it up. In fact, if the system is overcharged, it will not cool properly, either. Nor will it cool properly if the A/C condensor is plugged up, which is not an uncommon experience in the SW with the seasonal bugs that get lodged in there.