I'm trying to debug an existing NES emulator built into a N64 game - it's a long story.  There seems to be an issue though with horizontal mirroring, but in all honesty I'm not certain if that's really the case.

So, long story short, what would be the difference between these titles?

Working:

• Puyo Puyo
• Ice Climber
• Gyromite
• Stackup
• Donkey Kong, Jr. and variations thereof

Randomly Crashing:

• Excitebike
• Super Mario Bros.

It seems to be an issue handling horizontal mirroring, as when the crashes occur the screen is fixed in the non-visible swap area, never sprites except your main character displayed.   It is most likely to happen if you move quickly.  It couldn't be the mapper, since it shouldn't even boot if the mapper is unacceptable (it is coded to only allow 0-3)

If anyone has any clue what might be wrong, please let me know.  I only get to the internet infrequently, but I do eventually respond.