heating question

92 f150 4.9l

I am having a problem with my heat.

When my truck is at idle and sitting still my heater will cook ya like most Fords but when i start driving it goes to almost cold.

My temp gage stays right in the middle and does not change in proportion to the change in heat.

I replaced the heater core in the summer and thought it might be the cable not pulling blend door closed but it is hot at idle.

I know in the vent position on my Lincoln a door opens to allow outside air in to adjust temperature inside.

Does this apply to all Ford vehicles?

This summer the A/C worked great so if a vent to outside was open I would think the opposite would be happening. Cold turning to warm.

It is a very simple control as it is not electronic.

area of heater core stays hot to the touch yet blows cool air.

Would sure appreciate some feed back if anyone has an idea.
Thank you

I would be looking to see if there may be a vacuum leak in the system somewhere.

There's supposed to be a baffle that keeps flow going through the core. If it was not reinstalled when the core was replaced, then the majority of air flow takes a path of less resistance when underway (driving), which is around the core rather than through it.