My heater worked last October before putting in a better used engine. During the engine change all the hoses, check valves, water pump and thermostat were changed as well as fresh coolant being added.

The electric heater pump was very noisy. I replaced it with a used one that
was no better. I had my pump overhauled and now it runs quietly.

I replaced the heat control valve and had no change. I did find out the old one was good. Both the top and bottom water paths open with the heater on HI.

If I drive in Sport Mode and use full throttle acceleration, I get heat and it
will continue to work until the next trip.

Next time I drive there is no heat. I am guessing that one of the check valves may be sticking until the water pressure increases due to higher
rpm. Any other ideas?