A/C output goes down after driving for a while with A/C on

When I am using my A/C, everything works fine until maybe 20-30 minutes of driving. Then I notice the cold air from my vents decreases and keeps going down until virtually nothing comes out even though the fan is on the highest setting. It doesn't matter if the system is on MAX A/C or normal, or the outside temperature. I pull off the road and pop the hood and check the A/C lines and discover that they are iced up. I shut off the A/C and after a minute, I see water draining strongly under the car for at least two minutes, which makes me think that the evaporator or something else is freezing up causing the restriction of flow. Obviously, it's not really desirable to have to shut off the system every time this happens and I don't want to be without A/C especially when I need it.

The last time this happened, I took it to the dealership and they checked for codes and found that the compressor was faulty and was replaced while the car was still under warranty (over a year ago). All was good until just recently when my A/C started acting up again this week, so I really hope I don't need to replace a compressor again or something else now that the warranty has expired. I'm hoping it's just a case of the improper amount of refrigerant (over or underfilled) or something simple that I can fix myself. I can take the car into the dealership where I work and see what they have to say, but I don't want to hear the common "we couldn't replicate the problem- it worked fine for us" and sound like I am crazy, so I want to come armed with information and knowledge as to what to look for or test before I make an appointment. Do you have any idea what is going on and what I can do?