Firstly make sure you are playing from an Original game disk... and from experience, I found Splinter Cell (Chaos Theory from memory), crashed on my xbox at one point aswell... Could be the game coding, but also be mindful of how hot the console is... if you've been running it all day, it can freeze from the heat buildup.
Remember an xbox is a watered down PC (complete with pentium hardware, nvidia graphics card and worst of all, microsoft's name all over it), so its certainly not 100% problem free.
Crypticwolf, January 2006