My goodman furnace starts like normal but after a few seconds stops and then restarts a few seconds later it fires then stops then refires why

Try cleaning the flame sensor. That is the cheapest thing to do first.

I have the same problem with mine, I've done the cleaning of everything, even had a tech check it out, can't figure why it fires up and then shuts off, vent motor stays running, then resets and refires, sometimes going on for 20 minutes or so then finally staying on, be nice to know up else can be checked or done

The next step I would take is to measure the current from the flame sensor to be sure it is enough to satisfy the requirements of the circuit board to keep the unit running.

Have you checked the status light for activity? It should be visible through a clear viewer like this.