Swinging Temperature Gauge

[paulcnb]

Hi Fellas,
Around 6 months ago I was having trouble with an erratic temperature gauge on my 2003 ZT190. For no apparent reason the needle would leave the 9 o'clock position and drop one or two segments toward cold - sometimes it would drop to zero. If I stopped the vehicle and switched the ignition on/off once or twice the fault would temporarily disappear.

As it was difficult to determine the cause, I purchased a metal thermostat from Kaiser and installed it together with a new temp sender unit. I also physically checked the wiring to the temp sender which appeared to be OK.

Everything then returned to normal and after 6 months I was convinced that I had sorted the problem - until to-day.
When driving to the shops the gauge again began swinging from normal to one or two segments towards cold before returning to a rock steady normal position. This occurred 4 times in quick succession while driving in moderate traffic conditions.

For information, at the time of installing the new thermostat and temp sender, I also installed a digital temperature gauge - the sensor for the digital unit is attached to the engine manifold. As the readout of the digital gauge remained steady at 85 despite the intermittent swings of the dash gauge I'm pretty sure that the variations are not due to actual changes in engine temperature.

Not sure if it is important or not - but the needle swings very quickly when it decides to act up - like it would if the power was being switched on and off.

Advice from somebody that has "been there done that" would be appreciated.
Regards,