Low coolant?
System airbound and needs to be purged?
Water pump not circulating coolant?
Heater control valve?
This happens to me on my 99 at the start of each winter.
the damper "Sticks" in the shut position but eventually starts to working again after fiddling the climate control temp settings.
have you flushed your radiator or changed the thermostat it could be stuck
Have the heater core flushed. It might be plugged up with corrosion and dirt.
you need to have it flushed
The manufacturer recommends flushing the radiator every 30,000 miles. The radiator should be flushed once every two years regardless of the miles.
you need to have it flushed
You need to have the radiator flushed immediately
more than likely you need a new radiator for it, when you install it back flush the engine.
The radiator on a 1999 Mercury Cougar is flushed by draining the radiator, filling it with water, and adding a commercially available radiator flush. The engine is allowed to circulate for 10 minutes, drained, flushed with fresh water, and refilled with coolant.
Test gauge function with a mechanical gauge to get actual engine temp.
Either the water pump impeller is loose on it's shaft, or you have a leaking head gasket.
The radiator on a 98 Buick Century is flushed by turning the petcock to drain the coolant. Water is then flushed through the system until all contaminants are removed.
every 1-2 years