1997 Buick Lesabre heater only sometimes

Heater Core — Plugged

WARNING: The heater core inlet hose will become too hot to handle if the system is working correctly.

Check to see that the engine coolant is at the correct level.
Start the engine and turn on the heater.
When the engine coolant reaches operating temperature, feel the heater core inlet and outlet hose to see if they are hot.
If the inlet hose is not hot:

the thermostat is not working correctly.
If the outlet hose is not hot:

the heater core may have an air pocket.
the heater core may be restricted or plugged.
If both are hot, the temperature blend door in the heater plenum may be broken or stuck...

how do you fix the temp. blend door?

ADJUSTMENTS AIR MIX VALVE 1. Remove glove box. Start engine. Set temperature control to 90 °F (32 °C). Set passenger control panel to full WARM position. If ambient temperature is greater than 90 °F (32 °C), disconnect in-vehicle temperature sensor. Allow at least 45 seconds for programmer output cranks to reach full heating position. 2. Gently pull driver air mix valve crank toward programmer until valve contacts stop. Insert threaded portion of link into slot on plastic pin on driver air mix valve crank. 3. Gently pull passenger air mix valve crank toward actuator until valve contacts stop. Insert threaded portion of link into slot on plastic pin on passenger air mix valve crank. See Fig. 1 .