Fuel gauge sticking

I've got a 1998 Explorer, 4.0L SOHC with 120k miles. The problem is that the fuel gauge doesn't move as the fuel level changes while the engine is on (while driving). If I turn the key to the off position, then back on, the gauge will then show the correct amount of fuel in the tank at that moment, but won't change again until the keyswitch cycles back to the off position. Not a big problem for short trips, but for long trips you have to be sure to stop after a given number of miles to shut the engine off to allow the gauge to show a new reading. What could be the problem(s) and possible solutions? How can I troubleshoot?
Thanks!

* The fuel gauge and instrument cluster gauge amplifier must be replaced as a set. The fuel gauge and instrument cluster gauge amplifier are calibrated together.
* If the ignition key is left in the ON position during fueling, a slow to respond fuel gauge indication may result. The ignition key must be placed in the OFF position during or after refueling for accurate fuel gauge indication.

The instrument cluster gauge amplifier electrically averages the fuel sender readings so the gauge displays the actual fuel level and not the fluctuations due to fuel sloshing. The amplifier also supplies the signal to illuminate the CHECK GAGE indicator to indicate low fuel, high engine coolant temperature, or low engine oil pressure. The gauge amplifier is a small printed circuit board located in a pocket on the back of the instrument cluster. The electrical connections are made with a spring-type connector. There are no provisions for calibration or adjustment. The fuel gauge and the instrument cluster gauge amplifier must be replaced as a unit.

Hope it helps.