The fan viscous may be the problem as well as a plugged radiator or even a t-stat.
Check the radiator by running the engine till warm then shutting off the engine and feeling the engine side of the radiator for "cold spots". If present, replace the radiator. Check the viscous by running the engine and raising the engine speed for 40-50 seconds. The engine fan should increase and decrease in speed when the viscous engauges and disengauges. The thermostat is a good item to replace just for good insurance.