[jhv48]

Did you recently replace the gauge and sending unit or is this a problem that just cropped up? Possibly a dumb question, but if you did replace the gauge,did you pair up the electronic gauge with the correct sending unit? If so, then you might have a faulty sending unit.
When I wired mine, I ran the hot lead to a switched terminal on the fuse box, the ground wire to a grounded bolt, the lead wire to the sending unit and the last wire is connected to the dash lights.
The sending unit could also be sitting in/surrounded by an air bubble as mentioned. With the engine running, slowly loosen the sending unit a little until coolant runs out, then tighten it back up. That will ensure there is no air caught in the passage. If still no reading, call Nisonger and replace the sending unit.