If both heater hoses are full of water and are hot then most likely the blend door is your problem. Look up heatertreater on the net for more info and a possible fix to your problem.
It is possible that your heater system is air bound. Air inside the core cools very rapidly and hence the feeling of cold air out the vents. Air inside a heater is a no-no, it needs coolant to heat up the fans air which in turn comes out the vents.
Hope my confusing description is of assistance.
Good old Bill