Air condition performance

[asolde]

When running my air conditioner the temp of the air at the outlets goes between 60* and 68*. I have put a vacume on the system and leak check good, serviced it with freon with the engine running until the sight glass was clear reading about 38-40 psi on the guage. I weighed the freon going into the system at around 3.5 lbs. I replaced the compressor switch to begin with, replaced the water heater control valve. Replace the thermostat and fan clutch today engine runs normal 195* Seems like at lower speeds or at idle it runs the coolest,when I get up to speed the tempature goes up to 68* and back down to 60* or so depends. I would like to see 50* at all time at the outlets. Any Ideas of what is wrong?