Slipping clutch or software issue

I have a 2002 Ranger, 3L., Automatic, 4 WD. Ever since it was new it has felt like the clutch in the auto. transmission has slipped when shifting down from 4th gear (45mph)down to 3rd gear when gently accelerating. The truck makes 3 or 4 jerks before downshifting. If I push the accelerator hard, it shifts smoothly. This happens downshifting in all gears, but especially from 4th to 3rd at 45mph. Last January the local Ford Dealer changed my fuel filter and checked the fuel pump. All was okay. He replaced the transmission and drive line. 6 months later the same shifting problem developed again and he replaced the transmission free of charge. Now 3 months later it has returned! It can’t be three bad transmissions. And because it the engine doesn’t breakdown under a quick acceleration, it can’t be starving for gas or bad coil. Could it be a computer issue? I hate go go back for a 4th transmission! Thanks for your help.