98vw passat2.8v6 reach norm temp when idle but drops when driven. Reasoning?

What is your question?

I start and let idle heats up to normal temp and blows hot air. As soon as i get down the road a few miles the temp drops all the way down and blows cold air. Has coolant in system and no leaks. What could be the cause for this? It never over heats even during idle. Doesnt make any weird noises or drive weird. Overall is a great car

What makes this problem better or worse?

Heats car fine when sittin still. Cold when driven

How long has your 1998 Volkswagen Passat had this problem?

A couple months