Ah hah. I might be able to help here. I had this same problem when I first bought my S60: Clutch gap measures within spec, clutch initially engages then 'lets go' not to engage again until things have cooled off. Refrigerant charge was fine. I had resistance in the harness connector at the compressor.
The effect is the same as a large out of spec clutch gap: 'restricted' current through the wiring = lower magnetic flux. Once the engine/clutch coil got warm, the magnetic force decreased even more and once the clutch would cycle off it wouldn't re-engage until things cooled off.
My fix probably isn't the preferred way: I chopped off the connectors and soldered the wires together with shrink tubing over silicone sealant. Ideally you'd determine determine 'side' of the connection is responsible and replace it so as not to cause a headache should you ever need to remove the compressor for service. I decided chances are, it I ever were to need to disconnect the compressor, it would most likely be to replace it. If/when that's the case the new compressor will have a new plug. At that time I would replace the other end on engine harness.
Since then the car can achieve 'walk in freezer status' if I want it too. On a humid day I can get it so cold in there that condensation forms on the outside of all the windows and sunroof and I have to turn on the rain sensor to keep the windscreen clear. However I don't actually like it that cold. But the car can do it.
Do note, if resistance in the wiring is the source of your trouble, it might somewhere else on different vehicle- such as the relay or the coil ground wire etc-- or a bad clutch coil.