Your kind of bass akwards on your fan clutch diagnosis. Typically a faulty fan clutch will show itself as "overheats in stop and go traffic but runs cool on the freeway". I know it seems odd but a good fan clutch will typically have resistance when cold and spin freely when warm. The fan is mostly worthless at speeds in excess of 30 mph or so because the ram air going through the radiator is enough to do the job. Should it get warm enough that the fan becomes necessary is where the thermostatically controlled fan clutch comes into play.

From your description of the problem I would be more inclined to believe that you have a radiator that is externally plugged full of cat hair and other light weight crap. You may have a lazy thermostat or a radiator that is clogging up internally. Without looking at it I would say you more or less have two options, band-aid it by blowing the radiator out and replacing the thermostat to see if it helps or biting the bullet and putting in a new radiator and thermostat.

For a water pump to cause it to over heat it would have to fail to circulate coolant. Volvo water pumps are not without their problems but broken, slipping coolant impellers are not usually on the list of pump failures so I doubt that it is a problem with the pump.

Mark