I have the following issue with my 2007 Caravan. Just had the rad replaced cause it was leaking. Same symptoms, while idling temp goes up to normal range. While driving, winter in Canada, temp gauge drops back to almost cold, I can feel temperature inside dropping off. I have replaced the Thermostat twice in the last week and still the same above conditions, idling temp goes up but drops back to well below normal after driving for a while. Any suggestions?

 

For those who asked. The outside temperature here in Nova Scotia for yesterday hovered around the 0 Degree Centigrade

 

Are you sure your gauge is correct?
Is it the correct thermostat?
Meaning did they give you a 160* instead of a 180* or a 180* instead of a 192*. It might not seem like it would matter much, but trust me, it can make a huge difference.