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Based on what you describe, I would be looking at the temperature sender or some part of this circuit. It is my understanding that the sender feeds into one of the computers and there is not necessarily a linear relationship between temperature and indication on the dasboard display. If the sensor has become unstable and overly temperature sensitive, then this could be a cause.
Unfortunately, I cannot comment on what else could be wrong with the temperatre measurement circuit that could cause this type of response.
I wish I was more confident about this, but if I were in your shoes I would be looking at this next. One reason to suspect something in the measurement circuit is that I struggle to think of a way that the temperature could really increase so rapidly (ie in 5 seconds).
If the sensor has gone silly, it is possible that you are getting the response you see when running the cabin heater flat out as this pulls heat directly out of the water in the block, so the water presented to the sensor is marginally cooler and therfore the sensor reverts to a more normal reading. However, one could present a number of flaws in this logic, unfortunately.
If you had a portable digital temperature indicator, it would be possible to strap a thermocouple on somewhere near the sensor (thermostat cover?) and correlate indicated temperature to a real independant measurement.
Wolfgang
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