could be a series of causes, but I'm inclined to believe that it's rom-related.
this is caused by either a rom chip going bad (probably least likely, but still very possible given the game's age), the rom chip itself has crud on the legs (be it some kind of dirt or oxidation or whatever), or the rom socket has gunk in it too.
people's approaches vary, I've never tinkered with Nintendo roms before though, but I'm sure the removal procedure is the same. I personally use a long 1/4" craftsman flathead with the round base to sneak up under the ceramic part of the rom and give the screwdriver a slight twist, and pull it back out and go over to the other side of the rom and do it again.
you don't want to pull it out unevenly, or else you'll start bending the legs. not to say you can't go and restraighten the legs later, but there's only so much you can bend those until they break off.
once I've gotten both ends of the rom to at least break free of the socket, then I go back to the first side, slide the flathead back under again, and using my thumb to press down (not hard, just enough to hold it in place) on the 2nd side I feed the screwdriver through the full length of the rom (I push down the other end to keep it balanced so it doesn't bend the legs..) and by then you should be able to pull the rom free.
feel free to use a can of compressed air (use it properly.. don't shake the can, and make sure the initial spray is away from anything delicate to clear the nozzle of that liquid nitrogen looking shit first) to blow any objects out of the socket. don't blow directly downward, you don't want to lodge any shit in further.
chances are that's not the problem at all though. most users on here clean the legs on roms with a little pink eraser (not entirely sure if the gummier the better or if you want it stiffer?) and that usually cleans them right up.
try to do this one rom at a time so you don't goof up and plug them back in the wrong sockets. make sure the legs line up with the socket, if any one leg looks out of place, be sure to straighten it out first, you don't want to force the chip back in and have a leg bend in on you.
I straighten entire rows of legs by pressing them against a hard, flat surface.
I should just make a video about this!