i have my original NES and dozens of games from my childhood, and i have found that it is difficult to get them to run. thanks for the advice, i will give this a try!
I've found at times that starting with isopropyl alcohol doesn't get enough of the oxidation off for it to keep working normally more than a day or two, while the fiberglass pen being used once feels like a more permanent fix. (Maybe after 10 years it will have issues again, but it at least for sure keeps working for weeks or months). I had an issue of this sort really bad with a GBA power switch. You can try to get a little alcohol in from the outside and move it back and forth and maybe it seems to work for 1-3 days, but then it would be intermittent again. The proper fix was to desolder the shielding on the power switch to get inside, and then clean off the oxidation from the contacts inside.
I should've also mentioned this in my first comment, but I recommend opening up the cartridges before cleaning them. I don't think you can get at the whole contact properly while the shell is on, and it's easier to do an even up and down motion along the each contact when the shell is out of the way. I go back and forth for a couple seconds on each one until it looks shiny. As a bonus you can possibly find out if your game is genuine or counterfeit while it's open and the chips are visible.
You can also replace the socket on the NES itself for relatively little, it took me about 20 minutes a couple of years ago and it made it much more reliable.
You can also put the 72 pin connector in boiling water for like 30 mins, take it out and let it cool. That should reset the pins back to "factory condition"
If your socket is the unreliable part, I've found that getting a game fairly wet on the contacts with isopropyl alcohol and then repeatedly inserting/removing it will get the slot nice and clean inside. I usually blow it out with compressed air after that.
