Too many codes enabled at once can overload the GameCube's memory. Try enabling only one or two codes at a time.
Select Need for Speed: Most Wanted from the game list (or add it manually). nfs most wanted gamecube ar codes
Disclaimer: Action Replay codes are provided for educational and archival purposes. Using cheats may override legitimate high scores or leaderboards. Always back up your save file. Too many codes enabled at once can overload