• Yep check to see if the connections are loose if not generally it's the temperature sending unit gone bad on those

With age the instrument cluster connections get dirty and lose contact with the copper traces on the flex board. The connectors that plug into the cluster and the traces get dirty and stop making contact. 

I use a fiberglass brush from electronics repair. It will not rub off the metal but it will rub off the corrosion and the dirt. Dont use a wire brush as this could deposit metal on the board or other parts and cause a problem with the circuits or imbed metallic rubbings onto the plastics. 

You might be able to use a gritty pen eraser. The pink and white eraser that they sell for school supplies. They work good to clean these contacts and they are non conductive. Pink Pearl used to make one that was half pink and half white and gritty for erasing pens. Use that white gritty side of the eraser. 

If you need to find the temp sensor, it is on the intake manifold just below where the neck enters the air cleaner box on the lower forward drivers side of the air cleaner. It is threaded vertically into the intake manifold, probably where the water passes through the manifold. 

Check the connections of the sensor... But I am betting the connections on the instrument cluster are dirty which happens often for old nissan trucks. (I have one that has this problem and I have not got around to cleaning it myself. Did this before many times on other trucks) 

I have a service manual with the diagram of the sensor and its location for a 98... if you need it. Im pretty sure they are the same if it is a 2.4 liter dual cam motor.