Heater output cut off

I have a 98 Rodeo 2.2 liter 2WD that I purchased new which has had heater problems from day one, the heater only works when the truck is moving if you stop at a light to long or are seating traffic the heater just goes cold. Then when you start driving the heater output comes on mean while the blower is still blowing. It's like the heater core stops getting water or something. Took it to the dealer but they were not able to duplicate the problem. It did get a little better after I replaced the water pump, it would last a lot longer before going cold.