I've seen quite a few intermittent heat issues due to air in the heater core so starting with bleeding the cooling system is a great idea.
Start with the engine
cool and remove the radiator
cap. Make sure the coolant level is pretty close to the full mark.
Follow the upper radiator hose to the engine and you will find it goes to a plastic block that also has 1-2 other hoses attached and there is what looks like a brake
bleeder screw on top. This is the bleeder screw for the cooling system.
With the cap still off ,start and run the engine until it's at full operating temperature. Periodically open the bleeder screw while it's running to let air out that works it's way to the top, close the screw when a full stream of coolant comes out.
When it's at full operating temperature bring the RPM's up to 1,500-2,000 for 10-20 seconds and let it come back to idle, go back and see if any air comes from the bleeder screw. Repeat this a few times if necessary. Raising the RPM's will force more coolant flow than is possible at idle and it will normally allow more air to make it to the bleeder. There often isn't enough flow at idle to force air from the heater core.
Once you are no longer getting any air from the bleeder and you aren't seeing any bubbles in the reservoir then you can do a final top off of the coolant if it's low and install the radiator cap.
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