Sometime AC stops working

[TechMan]

I have XLi 2007 model and its AC was serviced last year. I am facing a strange issue that sometime AC stops working while driving. I mean one moment AC is working fine on setting 4 and throwing chilled air and then next moment I will notice that it will start throwing regular air. Sometime when I change from 4 to 2 then AC again starts working but sometime this trick doesn't work.
What could be the reason of this?

[shockxwave]

Could be a lot of things,
Could be bad expansion valve,could be a blockage in system,could be low refrigerant but most probably system wasn't properly vacuumed on last gas recharge which obviously would be the case if you had it refilled.Other than that there can be some electronic fault as well so better get it checked from a technician.One more thing could be Rpm sensor which gets the revs higher when A/C is on so that it doesn't trip (which I don't think is the case since it trips while driving).
Oh and one more thing it could also be normal tripping when optimal temperature in evaporator is reached and it trips momentarily to decrease continuous load on engine