You car is showing classic symptoms of temp compensator board failure.

http://www.ipdusa.com/pdf/PI-297TempBypassKit.pdf

The link is to the instructions for a kit to solve the problem.
If you have even modest electrical skills you can likely do the fix with just a piece of wire.

You can easily solder a short wire on to jump the two terminals indicated in the .pdf instructions.

Or you can do as I did:
Get a short bit of 16 ga. lamp cord. 18 ga. is too thin for this.
(cord usually has the gauge imprinted on the insulation, 16/2 awg)
Zip-split the wire, use just one of the two splits.
Cut 2.5 inches of wire, don't strip insulation at the ends.
Poke a thin nail up each of the ends to make a hollow.
Make a modest effort to clean the contact pins on the main circuit board.
Use needle-nose pliers to slip hollowed wire ends onto the pins indicated in the .pdf instructions.

The reassembled cluster holds the wire in place. No soldering, no nothing. Mine has been in place 22 months and 80K miles now.
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DAMHIK: Don't Ask Me How I Know - - - Sven: '89 245, IPD sways, electric rad. fan conversion, e-codes, 28+ mpg - auto tranny. 500 mi/week commute. '89 245 #2 (wifemobile). '90 244 (spare, runs).