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!
Back in 2000 about a year after I got my truck, I was experiencing some shudder. My local trusted transmission guy recommended this electronic deal that taps into the wiring for the solenoid pack inside the transmission. Its a circuit board that mounts on one of the valve body bolts. Its called TCC Shudder Fix. It really does work. My truck has 280k miles on it now and it shifts better now than it did when I got it. I think it was made by Autotrans or Transtar something like that. I was told that its now a company called Axiom.