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 !!!!!!!!!!!!!