Might not solve your problem, but I had the same problem with my 1990 NA with 253XXX miles on it.
It just started one day, the gauge would shoot straight to center as soon as I turned on the key and stay there while running.
Finally took off the cluster to have a look. Found a tiny nut loose in the cluster. The nut had come off from one side of the temp gauge. Put the nut back on nice and snug and gauge has been working fine ever since.