Yes, correct. The ambient temperature does have a direct impact on the ability of the electromagnet to pull in and grab the clutch. There is also an internal thermostat in the rear of the compressor which may get too hot and cut out the compressor. Both of these items can be fixed cheaply. First, the clutch:

Read from the start and go down to the "UPDATE" where he mentions doing the same to an S60, but using copper sheet metal. I used thin aluminum sheet metal, doubled over to get a reasonable thickness. I spread glue on both sides and with the clutch plate pushed in, slid the clip into place.

http://volvospeed.com/vs_forum/topic/80181-850-ac-clutch-gap-fix/

I used a pair of pliers to hold the glued shim and push the shim into place. Just fingers will not work. Remember, you can easily turn the clutch by hand.

If, after placing in 2 shims, there is any noise (dragging) as you turn the clutch, the shims are too thick and must be removed!! The clearance can go down to .018 or so.

Now, if the compressor still cuts out at 100F+, it might be the thermal switch in the rear of the compressor. Take a good look at the rear of the compressor for 2 black wires coming out of what looks like a 15mm bolt head. Follow both wires - they should each go to a connector. To bypass the thermal switch, disconnect both connectors and connect the wires that do NOT go to the thermal switch. The original black wires can be connected to each other, just to make it look cleaner.