1990 Accord - Engine Temperature Fluctuating

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My 1990 Accord (5 sp) engine temerature recently started fluctuating
as folllows : 1) start car and allow to warm up - Engine temp normal, 2) Put in gear and driive along on flat road, or go up hill - Engine temp normal, 3) Go down hill with car in gear, take foot off gas - Engine temp drops fairly quickly ( ~ 1 minute) to coldest point on the gauge 4) Engine temp comes back to normal going up hill
5) Engine temp comes back up to normal on level road, buit takes a little while after coming down a long hill

It's winter here, and cold outside. Heater is blowing warm air, even following a Normal->Cold cycle pattern, although, It could be warmer
when gauge is at coldest display.

Any ideas ??? Thermostat ???

Thanks !!!!!!!!!!!!!

 

Have you added, or changed the coolant lately? Is it low? If the answers are no, and no, I would check the thermostat, and the temperature sensor.