Heat goes cold at idle (98 Dodge Dakota)

Normaly, a lack of heat is often the first sign that indicates an overcooling problem, but in this situation, Your heater works great when you are underway but stop working at idle.
However, I'm also award that a lack of heat in the passenger compartment isn't always an indication that the engine is running too cold.
In most vehicles the heater core is located in the higher position than the rest of the cooling-system components. Because of its placement, the heater core is one of the the first component to run out of coolant when there is leak in the system. Therefore, when a coolant leaks exists, it's common for the driver to notice a lack of heat in the passenger compartement.
* A quick check of the coolant level tells you what's causing the problems. If the coolant level is good, check your thermostat.

Remove the Stat and check whether your heat increases at idle-if so change the Stat.

W/O the stat you got more heated coolant going thru the heater core therefore it should inrease a little. BTDT