Retail KOTOR crashing in modded xbox

[sirion]

I have a retail copy of Star Wars: KotOR, and it crashes either when attempting to load a new game or when trying to switch menu options to anything outside of "New Game".

I get the same crashes on my harddrive backup copy, and ***When my modchip is turned off***

And it works in my brother's unmodded xbox, which seems to rule out a bad disk.

What on earth is going on here? What can possibly differentiate my xbox from my brother's when the modchip is off?