Geeze, are you kidding? The thing will NEVER pull down to 75* if you keep opening the freaking windows and doors and shutting it down. Your Air conditioner is nothing but a big de-humidifier, it's going to pull the humidity out of the air before you notice a significant temp drop. How can it do it's job if you aren't letting it?
Temp drop across the evap coil will not tell you anything either if you measure it at high indoor ambient temp/humidity. You would think with a B.A. in HVACR you would know all this, I mean this is second month tech school theory we are talking about here.
A properly sized and installed system will run 100% of the time in full load conditions. Let the thing do what it is designed to do, and stop opening up the house and shutting it down!