1996 windstar bucking
 

If I understand what you mean by bucking, I would be suspicious of an ignition system problem. Usually, if it is starving for fuel it is a much slower power loss than if you lose spark to the cylinders which is just like turning the key on & off.
The 64 dollar question;;;; when were the spark plugs last changed, and you might be able to see in the dark if spark is jumping to ground at the coil.