I had the same problem with a Genesis cart. the only thing I could come up with was a corrupt ROM. there is a device where you can reflash the ROM or something but unless you have sentimental value with the cart or dozens of dead carts, your cheaper route might be to just buy the cheapest one you can find that works and swapping the guts with the one you have
the other possibility and you didnt mention it is the capacitor. however I replaced mine with a known good cap so the only other possibility is a bad ROM. my two cents. probably worth about that much too