Its time to step back and huddle. Two things I would suggest checking are;
(a)make sure the cooling system is properly bled. I have seen the bleed hose become plugged with casting material if the engine was severly over heated. With a cold engine, remove the reservoir cap. Remove the bleed hose from the radiator and blow into the hose. Air should pass freely through the hose.
(b) make sure the radiator is not partially plugged. The easy way to check this is use an infred red temp sensor and check the radiator temps from top to bottom, with engine at operating temp. The readings should be fairly consistent across the radiator.
Another note: If the center of the radiator is plugged it will be cooler than the rest of the radiator and not allow the viscous clutch to engage.
92 Volvo 740 Wagon aka "Mutt mover"