Joad, it looks an awful lot like this problem here (it's case one). I'd start by replacing the 74LS139 latch chip at location F2 and perhaps the one at E2, too. According to this, the 6502B processor could also be bad, or it could be a ROM problem, but I'd just start by checking 74LS139 - they're very cheap and easy to find enough that they're worth a shot.