I had the same problem and after a lot of investigation, i read someplace that I should check the gap between the coil and the clutch plate on the compressor.
In my car it was at 1mm. Correct gap should be half that much. What happens is this: With the wear of the clutch and with the widening of the gap, the magnetic field generated by the coil will allow for some slipping of the clutch, which generates heat. Heat on the clutch is transmitted to the coil. A hotter coil generates a weaker field and the clutch disengages, simply because the magnetic field is unable to pull the clutch plate, unless it cools off.

The correction is relativelly simple. What I did was to remove the serpentine belt, take out the screws holding the compressor in place ( without taking out the gas pipes and without degasing the system) moved the compressor a little so that I could remove the clutch plate and them removed a couple of the spacer washers behind it ( there are three) and reasembled) gap came down to .4mm . Worked perfectly. If you want to check if that is your problem, drive thge car with ac on until it stops cooling. Stop the car, leave engine and ac on, open the hood and very carefully tap the center of the compressor clutch plate with a heavy screw driver. If it engages, you found the problem.

Good Luck
Icelandic