When driving the gear revs normally but then randomly pauses/stops revving, then when i slam my foot down it regains power and revs up but has the same problem whenever i shift. Ive done a major service and replaced everything so it cant be plugs, leads, dizzy oil etc

I was just wandering, is the gearbox causing this, because the problem started the day after i got my g box replaced or could it have something to do with the engine?