I was having a problem with my temp gauge in my 85. It would read right in the middle upon startup (when cold) and then drop down to zero after a few minutes and then move up and read normally after the truck was warmed up. In the process of fixing my gas gauge (unrelated problem) I had the gauge cluster apart so I pulled the temp gauge and tightened the connections on the back of the gauge itself (two threaded posts and a nut, I believe).

It has been working properly since doing this so I believe I had a weak connection on the back of the gauge, where it mounts to the circuit board. You might try it if you have to take the gauge cluster out anyway.

Allan