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Minimania in the US has several articles on the instrument voltage stabilizer and how to replace the bimetal strip with a 7810 (and other) voltage regulator. BUT; you have to gut the existing unit and be reasonable at soldering and electronics. If you are not confident, don't!! ONLY use a 7810 regulator with negative earth. I can post one if you are still keen.
Several contributors thought that the airflow through the temp gauge might effect the reading but rapid airflow near the stabilizer might cool down its heated bimetal strip and alter its average output. This would then effect the gauges. Try taping some insulation around the stabilizer.
Another possible problem is a stuck OPEN (or missing) thermostat. (If so, the gauge might actually be correct. Use an infra red gun to check cylinder head and radiator temps. Mine stay near 80c with normal driving.
As you can see in the photo the 7810 regulator drops the battery voltage (12 to 15 volts) to about 10 volts. The metal backing is connected to the negative pin 2 leg and can be bolted to the earthed case of the stabilizer. Pin 1 (red) is soldered to the B battery connection and the Pin 3 (Lucas mostly used light green wiring from the stabilizer to bimetal instruments) solders to the I instrument connection.
I must admit that although the 7810 regulator that I fitted to my car works perfectly, it did not solve the problem of a poorly calibrated fuel gauge. It does give a stable voltage at the fuel sender that I have accessed for a digital fuel gauge with alarm. Dave D