Overheating after replacing Heater Core?

I didn't have heat and there was a syrupy odor in car. I replaced the heater core, and I had heat and odor gone. A couple of days later the car started to overheat. Replaced the thermostat. After a couple days I noticed a coolant leak. Replaced the lower intake manifold gasket, which took care of the leak. Then I noticed the overheating only while idling. Replaced the water pump.

It seems it always runs okay for a few hours but then it comes back. Radiator fans are working, the temperature sensor I also replaced. There are no signs of a blown head gasket. No misfire, no gray smoke from tail pipe and no antifreeze in oil.

I have bled the system many times for air bubbles, vapor lock. It seems they are gone but then come back. Any suggestions please.