How do I fix my heating problem?

I have an Iveco Daily on an 02 plate.
The heating works but not very well. During tick-over it will start to get hot and blow hot air into the cabin. However as i begin driving, at night for instance, itgoes cold again and refuses to get hot unless I drive the thing at 90 mph!!
In the summer its fine as the outside air temperature is higher but in these wintery months its a nightmare driving it anywhere as its freezing cold all the time...

Can anyone help please?????

Iggy
February 2009