AC problem
 

I recently bought a used 2010 Mazda 5. The ac isn't working right. I live in AZ so we get hot. From the get go, the ac just didn't get cool enough when the outside temp goes up. Tepid, almost luke warm air. It seems to be ok when it in the 80's (when you don't need it). It blows in the mid 40's. But as the temps go above 90 outside, the temp inside goes up and will blow in the hi 70's or low 80's. One weird thing is that when you press on the gas, the temp inside goes down. As soon as you let off the gas, the temps inside go up. Quickly. The dealer and his mechanic have gone out of their way trying to figure this out. They have replaced pretty much the entire ac system. New condensor, compressor, evaporator, expansion valve and pressure switch. There was a belt replaced, too, but I don't know which one. I am scheduled to go back in a couple days from now as they have ordered a new cooling fan. I was wondering if anyone else has experienced this issue and had any advice. I'm not sure what you call it but the control panel shows the ambient temp and the temp the ac is set at (going full blast it goes down to 60). I know the ambient one is mounted in front like around the radiator. Could either the outside ambient or the inside one be bad and some how restricting the ac? I read somewhere that even the car's thermostat could be an issue because in some cars it will restrict the ac if the car is overheating, but my car doesn't seem to be overheating. Is there anything outside of the ac system that could be causing this issue? Have we missed something? We are all flummoxed!!!