Why is my car not giving gas while driving?

Ok its a 2000 civic honda dx car runs but sometimes when I hit the gas it wont give power then itll go again then itll do that again well I had a friend/ mechanic check it out and first he changed the fuel pump then cleaned the injectors then he checked the distributor then he check catalyst convertor everything is changed well he gave it back to me its still doing that but only when I first take off for like a minute then it drives normal he hardly charged m e for this but still problem is not fixed it still has trouble going when I hit the gas so anyone know whst this can be??? I wish I csn tke it to a shop but im low on money right now