Nintendo Repair shop sells a two step cleaner system for game cartridges that I would really recommend you try. Your 72 pin connector may be fine but your games may have gunk or corrosion on the connectors. I really don't recommend using peroxide or sand paper on the 72 pin connector. In my experience once a 72 pin connector is shot it isn't because of corrosion it is because the metal has worn out and it is not making contact with the game because it has lost is spring to make a solid connection.
Don't use games that have not been cleaned on a new 72 pin connector.