John, I have had 3 of my SPA gauges do this same thing. Fuel Pressure, EGT and now rear diff temp. What I finally did that worked for the first 2 was to cut off the factory connectors and just solder the darn sending unit into the wires from the gauge. That's drastic I know, but it worked. Except for the rear diff gauge... I am going to call SPA and get a bew sensor to see if that cures the problem.
Tom