1989 Chevy Truck Radiator fan always engaged

I recently noticed that my radiator fan was remaining on all the time. As a result, the fan clutch actually got worn out to the point of wobbling really bad, and I replaced it. It operated properly for about 100 miles (Engaged sometimes, but most of the time was disengaged). Now, it's back on all the time. My thought was that maybe the engine is running too hot, so I topped off the radiator(it took about a gallon of coolant). It's still on all the time. The fan clutch is completely mechanical with a thermostatic spring which engages and disengages the clutch according to direct temp. This tells me that my engine is still running hotter than it should. Could this be a faulty water pump? Or thermostat? Something is preventing my engine from totally receiving it's cooling resources. (Note: I have not overheated at all, and the temp gauge looks normal all of the time) Also, there was one occasion when the temp gauge did rise rapidly at a red light, but then fell again just before overheating (this was before replacing the fan clutch). That only happened once to my knowledge (could have happened again, but didn't notice it). Any ideas as to why my engine is hot enough to keep the fan clutch engaged constantly, but never hot enough to overheat?