On the back of the speedo there is a aux circuit board called the Temperature Compensating Board, the pins that attach it to the board can and will get grungy and a simple removal and cleaning can sometimes fix the problem. Also check the yellow wire from the Temp sender at the left front of the block to the gauge, it passes under the intake manifold and can get toasted so the insulation falls off and it can short against the block or head...the airflow under the hood can move the wire around so it will short-stop-short-move-short...see what I mean?

Start with locating the yellow wire at the firewall connector and attach a new wire to it w/ appropriate sleeving and pull it back to the sender using the old wire as a leader. Then attach a spade connector, hook it up and see if the problem goes away...then if its still happening do the Temp board fix...