Curing 1996 plymouth voyager 3.0 liter engine flat spot hesitation acceleration

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whenever I accelerate after underway, my 1996 Plymouth Voyager SE with 3.0 liter 6 cylinder engine (fuel injection, mileage:120,100), hestitates (flat spot), especially between 50 to 80 mph speeds. I must ease off very gradually, until it finally starts to accelerate again! What should I do in the proper order of steps (example: 1 thur 10 steps) in order to cure? What about a Fuel Injector Fuel System Cleaner - added to gas tank? Or just what is best to try... First?..Second?..Third?....etc?? Thanks Folks! Tom, Adelphi, MD ..9-17-06.