1993 Dodge Dakota Heater doesnt work

I read a response about heaters not working, but mine will go on and off frequently. For example: I start my truck in the morning to warm the engine up. I enter the truck about 25 min. Later and the guage showes the engine is warm, but the air is ice cold. When the truck starts moving, it quickly becomes very warm. Then after maybe a minute or two, it becomes icy cold again. Then, naturally, I shut off the vents. After a minute (while the a/c is still off) I feel a very small amount of warm air coming through, so I turn on the vents again to get all the warm air and it turns icy cold again. Any idea as to what it could be specifically? The radiator is less than a year old ( the old one had major cracks, so I bought a brand new one)