2000 ranger automatic transmission

I have a 2000 Ranger with a 3.0 engine. After the truck sets over night, the transmission shifts great until I drive about 15 miles then it starts to slip between gear changing and there is a delayed downshift back to first when I come to a complete stop. I have changed the fluid and filter but am having the same problem. Is my tranny going or is there a sensor that can cause this? The truck has 85,000 miles on it now and I have never towed anything. Any advice before I bite the garage bullet?

is your o/d light coming on without pushing the o/d button? mine had the same symptoms at 42,000 miles and it was the valve body gasket deteriorated and clogged everything up. $1800 to rebuild.get it to a shop before you fry more internal parts.

no od light. Someone said it could be the transmission range sensor. Will check into that.