DIY live has some good tips on fixing that old NES “I took apart my nintendo, and found the 72 pin connector. I took my smallest flat head screw-driver and pried every pin up a little. Over time, they had lost their springiness (I don’t know if that is a word, but it describes what I am talking about). This is a pretty good fix. It may not be the best, but it works. Another thing to help out is to clean the pins of your old cartridge. You can take a wooden pencil with the red eraser, and rub it on the pins of the cartridge. Make sure you get all of the eraser off of the cartridge.” Thanks Star! Link.
