on my 96.....in front of engine on top of water pump is the temp sensor that feeds the computer and tells it how to adjust fuel/air ratio....there is a temp sender that feeds the gauge/dummy light.....it is located at back of engine on driver's side beside the valve cover, screwed into the intake manifold, in between the fuel rail and injector harness rail...see pic below...check the electric fans connector behind the rad...it could be bad or have a bad connection....electric fan only comes on with a/c or when you hit ~220 deg....
....how did you test the sensor in the water pump (thermostat)? if you disconnect it and check resistance across the sensor terminals....you should see very high resistance when it is cold and low resistance when it is hot/warm....your 99 engine may have a combo sensor/sender! i am not sure.
...here for more renix engine tips, but they still apply for troubleshooting.... http://www.lunghd.com/Tech_Articles/...iagnostics.htm
..... "Basically - this sensor functions by providing a resistance-to-ground circuit that the computer (ECU) uses to determine how much fuel the engine requires. (The hotter the coolant - the lower the resistance.) The resistance of the sensor is read by the ECU which in turn adjusts the air/fuel ratio as close to 14.7:1 as possible. When a coolant temperature sensor fails (open circuit / no continuity) the high / infinite resisitance will cause the ECU to falsely read the max low temperature of -40 degrees F. The ECU then tells the injectors to enrichen the fuel mixture because it thinks the temp is really cold when it really isn't. A short to ground would cause a 'no-resistance' fault causing the ECU to think the coolant air temperature was extremely hot and lean out the fuel mix."