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.
Hmm interesting, I too have repaired many NES systems but the only thing I can think of that is causing this is a connection problem. What type of connection are you using to connect to your TV? I recommend a composite video cable, you can grab one for a buck on ebay. Other then that I would reccomend you take your NES apart make sure the 74 pin connector is solidly connected. I buy all my stuff from Nintendo repair shop, they have quality parts and they also have stuff to clean your games. So far I have not had a NES system or game that does not work. Way to go Nintendo for building solid machines.