Ignition or something????

My 4.2 liter has been constantly dropping out on normal ride on the highway about every 15sec. makes itself felt as if the motor braked runs but in the stand impeccable. And when he starts, he does not accept gas, and it is not until after a thoughtful moment that he is full. it starts perfectly and runs otherwise great only the fluctuation of the speed in the partial load range and when starting the triggers are not to explain knows there is someone advice? Greetings andreas