Sonic the Hedgehog 1 (Sega Genesis) ESRB variant

[billybob884 56]

So I just acquired a copy of the ESRB variant for the original Sonic the Hedgehog (Genesis), and while it's physically in good shape, the game won't boot properly. It usually reaches the title screen, where it improperly loads the tiles/colors and crashes to either an "illegal instruction" or "address error" message, or sometimes it will actually load into the first level, but doesn't actually load the level, so Sonic just falls to his death 3 times (then the game crashes on the game over screen). I've cleaned the pins to a mirror shine, and tried reflowing the solder on the chip's pins, and I'm not finding any continuity problems on the traces, so my guess is the chip has failed (though from what I know, this is still pretty rare).

I've got plenty of spare Sonic 1 copies to swap out the board, but for the sake of accuracy, does anyone know if the ESRB copy is a ROM variant?