'98 Explorer AC Problem

Hi - here are the symptoms of my AC problem:

- AC is fairly cold when idling.
- AC is lukewarm to cool (but never cold) when vehicle is moving (faster speed = less cold).
- Fan motor and heat settings work great.
- Vent blows warm-hot.

I've had the car about 1 1/2 years and the AC problem was existing. My mechanic recharged the refrigerant and added sealant a couple of days ago, but it did not fix the problem.

My Explorer is an Eddie Bauer (5.0, 2wd, auto) with the push button climate control center. Problem described above occurs with either the "Max AC" or "Automatic" buttons pushed and the temp set to 60.

Father-in-law mentioned something about a blend door or related motor or ductwork. But I just never learned about AC systems.

Looks like I'll need to put it in the shop, but I'd appreciate any comments/insights before I do.

Thanks,

John