heating system

Sounds like your heater core is not flowing, so you need higher engine speeds to pump the hot coolant through, a simple first step would be back flush your heater core, that would eliminate a very important factor. Then make SURE that your hot coolant is flowing through your heater core, by feeling the heater hoses. When the engine is hot, the upper and lower heater hoses should be hard to hold. If the coolant is flowing through your heater core and it is not plugged up, then you may have a climate control head that is shutting off the heat when you are at idle for some reason. Move your temp control when there is heat flowing and see if it changes and then move it when you are at idle and see if it changes. I am assuming that your blower fan is working correctly yes?