The NES is a very reliable system once you get to know it. Basically, rubbing alcohol is your friend. Use it to clean games and systems inside and out, just not labels/stickers you want to keep. A simple cotton swab dipped in alcohol is the preferred method for cleaning games. To clean the system, you'll have to open it up, but don't worry, it's very simple.
There are a multitude of guides out there on how to restore the 72 pin connector, but the one I used originally and still prefer to other ones is
this one. I would like to add that when you have the connector out, take an old toothbrush dipped in alcohol to clean the connector as well.
And if you still have problems with games playing especially if it's just one or two games that refuse to play, you'll have to disable
the lockout chip.