Yep, the Xbox 360 is nortorious for save game issues if you overwrite your saves.
Here are other games I have played on the 360 with the same problem.
Fallout 3
FarCry 2
Sacred 2
F.E.A.R
Brothers in Arms: Hells Highway
And these are just the games I have had this issue of overwriting saves, which I do not overwrite anymore, and problem solved!
