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Possible problem: Memory overwrite.
Use memory profiling tool to find it, like insure++ or purify etc

It also depends on the application and the way it is running. If you run the whole application and not particular event then it may cause the problem as there may be more than one module interacting.

This might be because of memory leaks .
This might be either because of heap overflow or corruption of data on the stack .

For detecting these issues , we can either use Purify, calgrind and other dynamic memory watch tools.

If there is memory leak or overwrite, it should crash at the same place all the time. I think there is a random number/some variable which is deciding the behavior of the program/crash site.

double deletion of pointers. Once a pointer is deleted the memory is freed and the program can allocate the freed memory to some body else.. this happens in multiple threads case where you might end up deleting a ptr in two different threads and depending on whom the address pointed to by this deleted ptr got allocated the crash might show up in different places..

Some reasons for crashing in different place each time:
(some overlap, some are similar, there could be plenty more)

1) it uses random numbers.
2) it uses time.
3) it persists state and reads it back the next time.
4) it could be eating up memory. Depending on other processes in the OS, it could get out of memory exceptions at different times.
5) the application is trying to mess with the OS (like trying to set the pagefile size) (the C standard library restriction might remove this cause, though)
6) it is trying to read/write to hardware device. (again, not sure what C library gives).
7) it is using shared memory.

if the source is given, why not show it here? It would be easier :-)

Jack is absolutely right. I have seen such issues many times in my 4 years of experience. The major cause is HEAP corruption. Corruption in stack (local vars) is never that unpredictable, unless coupled with BO.

Apart from heap corruption and other fun stuff,
what about more basic stuff like - uninitialized variables + the "not all paths return value" scenarios ?