I'd bet dollars to donuts the insulation on the temp gauge wire has turned to mush and the wire is shorting out. If you haven't replaced the five wires coming out of the eight pin grey plug located at the center of the firewall, it is time you did. Where's where the wires go:

upper left pin: brown wire (fat); coil terminal 15
down one: not used
down two: not used
lower left pin: yellow/blue; starter solenoid
upper right pin: not used
down one: red; alternator
down two: yellow; TEMP SENSOR (this sends a siginal to the gauge, not ECU)
lower right: black; oil pressure sensor

Cut the old wires off about 2" below the plug and solder on five new wires six foot long, 14 gauge stranded copper with oil and heat resistent insulation. Use heat shrink tubing over the solder joints. You can unplug the plug and do the soldering on the bench. Route the new wires around the inner fenders to their destination. Cut the wires to length and use the proper crimp-on connector on the end. Leave the old wires in place. This wiring job is eaiser if the intake manifold is removed first (good time to clean flame trap,breather box, and associated plumbing). This is a great holiday project if you have a heated garage and a couple of days off. jp