[Nick_R_23]

Sounds like you have a wiring issue. In simple terms, all those sensors are is a heat sensitive resistor. The more the circuit is closed (or opened), the more the gauge will move. If you say the block is well grounded to both the body and the chassis, and the sensor is well grounded to the block, then I would be checking continuity from the sensor wire to where it connects to the gauge cluster. If the wire and the circuit board for the gauges checks out OK, check to see if the gauge cluster has a proper ground. I believe this gauge also has a 12V feed, iirc. If the ground and power to the gauge is also good, then the gauge would likely be faulty.