My '96 E150 has been "stuttering" between 25 and 35 mph. Holding it steady at those speeds is the only time I get that reaction. After much mis-diagnosis from different people, I may have stumbled on something that works.
I was told by a former Ford dealership employee that Ford knows about the problem and sells a product designed to address it. It's call "Shutter Shift" and is only available at the dealership. It's a liquid that is added to the transmission fluid.
I'm changing the filter today and adding the product (provided my dealership has it). I'll report my findings after a few days of driving!

Tim
1983 F150
1996 E150
Lynchburg, VA