Had similar issue with cold air - turned out to be Head Gasket.
Here is my post: Front Heater Problem
Get your car checked at local radiator/service - they might do test for free, they did for me...
Few things you can also check:
does your coolant level drops sometimes?
if you take radiator cap off and start car does it pushes out air bubbles?
does temperature gauge goes from hot to cold very fast when engine is running?
In my case in the beginning I didn't have a heat and coolant level wasn't droping for first few months, but after one ride to local ski resort (additional load on engine...) it started to show all the BAD signs - coolant level dropped and engine overheated.
As I said - my suggestion - get you van checked for blown head gasket.