Cadillac Repair: heating, cadillac deville, heater core

Question
when you sit still it gets warm but when you move it gets freezing i have 1990 cadillac deville

Answer
IS or HAS the check engine light ever illuminated while driving the vehicle ?. If it HAS, I need to know.

If it has NOT, Raise the engine speed up to a fast idle for about 15 minutes and feel BOTH heater hoses where they go thru the firewall and one should be extremly hot and the other slightly cooler, Are they ?

In 90% of most cases the fault is a blockage in the heater core that needs to be replaced.