Just rip your NES apart, clean everything, and I mean everything. You'll be amazed at how much crap can build up on an NES mobo after 10 years. Same thing happend to me and it still fucks up the odd time, my suggestion is to take off that little metal bar where the game goes, and stick another cart over top of the one that's loaded, or get yourself an NES game genie.

Or go to a pawnshop and buy a replacement deck for 10 bucks.

I came to kick ass and chew bubblegum, and I'm all out of bubble gum.