What have you done in troubleshooting the problem?
I suggest taking a singular 200 to 400 ohm resistor, any value in between, and attach one lead to engine ground, the other lead to the green wire going to the gauge without the sensor and view the indication.
If rock solid indication, then bad sensor.
If the indication still varies, move resistor to the gauge itself and disconnect the green wire and connect the resistor.
If gauge indication is solid, check the green wire for chaffing to body ground and firewall connector for good connection.
If indication still varies, bad gauge, or bad ground on gauge itself, or intermittent power to gauge.
The problem may not be power as you did not say any problems with the other gauges as each gauge power comes from one wire in the console harness.
Could be a bad crimp in the power wire for the gauge, depends upon how the harness was made.
Unless I'm missing something here.