For what it is worth: I had the very same problem for almost a year. It was very frustrating. After my game crashed it would always repair some files. I could never tell how long it would play before it would crash.
I happened to be trying a new program called MemTest showing a friend how it worked when I noticed my memory was failing the test???? I never thought to check my memory! I bought some new memory and replaced the bad sticks. I have not had a crash since. Well except for the crazy MA mission crashes.
