Is the cheat file corrupt? Or maybe the file system is getting fragmented, and the programming that reads the cheat file may not support fragmented file reads.
So a couple of suggestions to try:
- Defragment the storage medium (I'm guessing SD?)
- Format the storage medium and include only the ROM, cheat file, and maybe save file.
- Validate the cheat file is in a good state.
- Create a new cheat file containing only the cheat you are interested in.
- This one is obvious but ... look for firmware updates or updated patches for the ROM.