I got your email, I just got back from a vacation in Canada.
I would tend to think along the lines of the evap temp sensor also, your mechanic could watch the actual value of the temp sensor with his computer to verify the reading. Sometimes the evap temp sensor will give a false reading and the control module (which is the actual part with the control buttons on it in the dash) will assume the reading is correct and not set a code. So if the evap temp sensor say "hey, it's -40 in the evaporator section!" the control module will shut off the AC compressor, thinking there's no dire need for any lower temp in the evaporator box, it's plenty cold, too cold in fact. The repair id to replace the evap temp sensor, which is fairly easy, so it only gives the correct reading.