KingofOSU

Hey all, I just put together my old system from when I was a kid. I brought all of my games from back home and got them all to work but one. My Donkey Kong Jr. and some other games don't work unless I hold them tightly into the console. Is this a common problem? I got some games off of E-bay that are the same problem. If this is a common problem, is there an easy fix? Is the cartridge just dirty or something, and how do I clean it? Any help you guys can give would be greatly appreciated. Thanks