yeah, both A/C usage and known to be inaccurate/intermittent coolant temp guages would make this kind of behavior noticable.
if you're convinced the dash guage is correct, go steady-state at a decent speed(~50-70 MPH would be a good target), with the engine up to full temperature, engage A/C for a couple of minutes, then turn it off for a couple of minutes. see if there is a noticable change on the guage.
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