ok so I have been having the same problem of the game freezing within 5 to 30 min on any mission but (knock on wood) I tried this method and it has been working so far after 3 hours of game play with no freezing.
1) Delete all game play, marketplace data from your xbox hard drive (move your saves over to a flash drive first) I have a 14g rather new one. I also deleted every other game on my hard drive so that it was completely empty (not sure if that matters)
2) Delete the system cache
3) Reinstall the game on said flash drive, but DO NOT download any updates
4) Do not turn off the system after the install and do not install disc 2
5) Insert disc 2 when instructed and play!
So far this has worked for me, it might not for you but just trying to help as this problem is seriously frustrating......good luck!!!