Okay so i have a 2000 Ls400 and whenever the car is not moving the heater starts to blow out cold air. but as soon as i rev the engine or start moving the heat begins to come again. anyone have ideas on why this would happen?

 

Thermostat, water pump, partially clogged heater core, low on coolant....

 

of all those it's 95% going to be having an air bubble in the coolant.

 

do a search...many MANY posts on this subject, most likely just burp the air from the cooling system, and all will be go...................