Most of the time the AC is great, whether driving, stationary, extreme heat, etc. Inexplicably some times, after an hour or so of running, it drops in effectiveness, air from the vents is still cooler than outside air but not as cool as it was previously and not cool enough to keep the car cool. It is as if the compressor is on half strength. Over-riding fan and temperature settings (ie set fan to full and temp to 'LO') does nothing to improve matters. Switch off the whole cooling system for a couple of minutes and it is back to full effectiveness again. Then after an hour or so ... and so on. Not great for long journeys on hot days. I have not yet been back to my local service department.  I will but thought I would ask here first.

I have had my system fail after a long trip but also the air flow through the vents slows up. This is due to the evaporator freezing up due to a problem with the evap temp sensor .   A overcharged system would also give the same problem if it freezes up. If the airflow is still normal the fault could also be due to moisture in the system which is freezing up 

 

look for fault codes with the climate control first with forscan