Temp needle weird behavior.

Hi,
Buick Century Limited, 1989, 3.3, 300.000km.
Starting cold, temp needle going up to middle (105F) and then suddenly, going down to 1/4 of the range, and from there it will going up and down
between 1/4 and middle of the gauge till it stabilize over time to middle between 1/4 and 1/2.
Cooling system full, plenty of heat in car, when it happen, less heat is coming but will eventually get warmer with the needle rasing again.
Radiator fan is working if this matters.
Any clue? themostat perhasp?
Tia
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-snip-
Sure sounds like it-- And thermostats are generally real easy to replace & relatively cheap so it would be wise to replace one that seems to be sticking open causing no harm [other than poor fuel mileage & lack of heat] before it decides to stick closed and overheats something.
Jim
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