Honda Civic '98 - Heating Issue

It’s freezing here in Michigan and the heating in my car acting weird. If I idle then the engine temperature gauge shows the correct reading and hot air blows into the cabin.

Now, once I start moving and get up to speed, the gauge starts dropping and in a matter of seconds it goes down to “C”. It barely blows warm air in to the cabin.



Now is this due to a faulty thermostat? or has the coolant become old and gunky? The coolant level is OK.



Any help would be appreciated. Thanks

Sounds like the thermostat.

Thermostat