[Millennial Toolbox]

Did you change the water pump with all those hoses and other cooling parts? Maybe its a little weak and not moving the coolant enough untill the rpms are raised, and having a proper mix of coolant and water is important to, to much water in the mix might not retain the heat throughout the system as much.

 

[Millennial Toolbox]

I agree with Sapote, a plugged, or "sluggish" heater core might be it. You should be able to flush it with a garden hose and attachments.. Or you can reference my youtube video where I physically remove the heater core and replace the 2 o rings that seal it to the metal pipes. When I actually had it out of the car I flushed it with a garden hose.. not with crazy pressure or anything, just sealing a garden hose between my hand and the inlet of the core, 2 years later no problems and my cooling / hvac system basically functions like new.