It takes a LONG time for my heater to warm up… to be expected of an old car. However, once my heater is warmed up enough, it blows cold air when I’m idling at a light or anything. Once I get back up to 35 MPH it goes right back up to its temperature. Then drops again when I stop.
When was the last time you installed a new thermostat?
Have you confirmed that the cooling system is full by checking the radiator (not the overflow reservoir) when the engine is cold?
Have you had your mechanic “burp” the cooling system to get rid of trapped air?
Your symptoms sound like one or both of the above situations is the root of your problem.
An old car should warm up just as quickly as it did when it was new.
Along with a new thermostat and a check for air in the cooling system, you may want to look for a vacuum leak in the HVAC system under the dashboard.
A malfunctioning vacuum controlled blend door (which regulates interior air temperature)could do exactly what you describe.