WinApe emulator. OK I've done further testing... it doesn't work correctly in WinApe, but using Ace-DL it works fine.
If you're using WinApe, the problem might be due to using the 464+ ROM, because the game is Basic 1.1 (the one used by GX4000) and you should use the 6128+ ROM. This is just to rule out errors.
