04 Impala 3.4 cooling issue

behavior I see:

engine starts and drives fine

temp gauge at start is low progresses up when cold and warming up as normal

temp of heater or defroster is cold even when temp gauge progresses up

when engine temp hits about 200 the air from heater/defroster warms up instantly

engine temp then starts to drop down to about 180 where everything gets cold again and temp begins to rise to about 200 where it warms up and starts dropping....

this cycle just repeats...

Check coolant levels. Thermostat or bad cap.

In addition to what the Kat suggested, feel the heater core hoses....when you have no heat, BOTH hoses should be hot.....if outlet is cooler, then you either have an air pocket due to a leak, or a restricted heater core....

If both are the same temp, check heater door operation, especially if auto temp.....