I had this once with a job lot of games from yamatoku classic.
what fixed mine was a metal polish, I really really don't like using this sort of thing on games but I didn't go out and get some brasso or some other caustic overly abrasive stuff, I had a jug of this:
http://www.malcoautomotive.com/Metal...h-details.aspx
whats interesting about this stuff is that its made to remove the stuff off of the surface of the metals, rather than get scratches out of the metal so its a very very very fine abrasive, finer than any thing off the shelf at Walmart.
I rubbed the contacts till they were shiny like new and it worked after.
I think a lot of people avoid the metal polishes because most of them are highly abrasive and start causing more problems than solving, this stuff really does the trick if its more than what an eraser can do.
now I don't go polishing all my game contacts with this stuff this is just the last resort if I cant get my hi-polymer eraser to do the trick, 9 times out of 10 the eraser works