2001 Toyota 4Runner Engine Misses

Only when accelerating the engine misses. At first it did it maybe once a week. Now it does it anytime you accelerate. If you apply constant pressure to the gas pedal, engine runs fine. As soon as you apply pressure to the pedal it misses. If car is sitting still in neutral and I race engine I can also feel it miss. We bought the car with 70,000 miles on it. We have not doen a tune-up since we have owned it Should I try additive to fuel tank before I try anything else to clean injectors.