Intermittent heat usually means bubbles in the system, thermostat sticking open, or a problem with the dash controls.
Every time you go out to the car for the first trip, when it is completely cold, open the radiator and confirm it is staying full to the top. This is by far the most common reason for intermittent heat.
To check and flush the core you would disconnect both heater hoses from the engine and try to send water through the core from a garden hose. Remove the hose nozzle, open the faucet fully, and hold the end of the hose against the end of one of the heater hoses. Do this in both directions. You should be able to send the full flow of the hose through it with very little back pressure. A car's water pump is not designed or intended to make a lot of pressure to push the coolant through restricted passages.
Every time you go out to the car for the first trip, when it is completely cold, open the radiator and confirm it is staying full to the top. This is by far the most common reason for intermittent heat.
To check and flush the core you would disconnect both heater hoses from the engine and try to send water through the core from a garden hose. Remove the hose nozzle, open the faucet fully, and hold the end of the hose against the end of one of the heater hoses. Do this in both directions. You should be able to send the full flow of the hose through it with very little back pressure. A car's water pump is not designed or intended to make a lot of pressure to push the coolant through restricted passages.
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