A) Test your cooling system pressure cap. It should not leak down and should release pressure above 17psi.
B) Install your new thermostat.
C) Test the fan clutch operation. You should hear it roaring when accelerating from a dead stop when the engine temp has climbed up around 105c.
D) If you suspect the fan clutch is not working turn the engine off and measure the radiator temp with your hand or some temp measuring device. The most crucial part is the core near the cooling fan. If its cool to the touch then the radiator is stopped up inside.
E) Make sure the fins are not stopped up with debris.
92 Volvo 740 Wagon aka "Mutt mover"