Sounds like a bad resistor in the guage. I'm not experienced enough with these to know whether they are RTDs or thermistors, but it appears from the other posts/threads that they are thermistors.

Regardless, they are likely read using a wheatstone bridge. The circuit will have a constant voltage supply and three known resistors. If there were a problem with the voltage supply, the error would change as the reading changes.

Since the error is a constant number, that only leaves the wiring and the internal, known resistors. If you can read the resistance where the leads go into the guage and the value is the same as it is at the sensor, it must be one of the internal resistors. One of those in particular will give a step change in the reading.

Here's a good wiki page if you want more info:

http://en.wikipedia.org/wiki/Wheatstone_bridge