Do you have a purge kit? Do you purge at all? It could be going momentarily rich when you first hit it because of air in the nitrous line. Try putting the car in neutral, rev it up to about 5k rpm, and hit the nitrous. The motor will rev up on its own. Do this a few times and then take it on the street and see what it feels like. If there is no more bog, you just need to purge before you hit it.