240's after '85 or so are prone to a bad circuit board in the instrument cluster, this is the temperature compensator board.

Consequence of failed compensator board is that temp gauge needle reads incorrectly and sometimes erratically.

Failure usually shows up as reading high, but sometimes can read low or eratic up and down. Sometimes reads high with headlights on. Sometimes drops if you turn on cabin heat and fan.

Temperature Compensator Board goes bad.
Can be replaced.
If replaced, will often go bad soon. Been there, done that.

Solution: Remove temp compensator board and bypass.
ipdusa.com sells a small kit to do this.
Also you can go to ipd's web site and find the instructions for their kit, as a .pdf file. It's quite helpful (hint).

If you already know how to remove cluster, compensator board is on th3e front, immediately below the temp and fuel gauges.
Release the tabs on ends to remove the compensator board.
"Virtually" number the connector pins left to right, #1-4.
Jump pin #1 to pin #3.
Soldering works, but see just below for a much simpler method. Proven through winter, 1/2 a summer, and 35K miles of driving.

I've done the job three times now.
I used a small piece of 16 gauge lamp cord, about 2.5 inches long.
(single wire only)
Cut the wire ends straight through the insulator and the conductor,
without leaving the usual bare wire protrusion.
Poke a sharp tool (like thin sharp nail or a metal punch w/sharp tip) into end of cut wire to leave a thin deep recess that you can slip over the previously mentioned connector pins.
That's your connection.
No soldering, no special connector terminals, etc.
Structure of the reassembled instrument cluster keeps the wire from slipping off.

So far, I have about 35K miles on the first car I did it on, since the bypass.
Still working fine.