From what I've been able to find your RPM sensor in that vehicle sends a signal to the PCM, which does it's thing with that data, and then sends a signal to the tachometer telling it what to display. Most likely it is an issue with the gauge itself, as you'd likely experience other issues if the PCM or RPM sensor was malfunctioning.
Just to be clear, the vehicle is running normally just the tachometer is acting up?