93 escort LX - temp guage acting goofy.

170K miles. temp guage does not register (needle sits below 'zero' line) until engine is warmed up *and* accelerator is let off *and* car
is not slowing down.
for example, letting the accelerator off on a slight downhill on the highway causes the needle to swing up to normal op temperature over the course of a few seconds. as the car slows, it tends to drop back down to below the zero line.
any thoughts?
thanks, anthony
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