Migration C++ project from Visual studio 2008 to VS2013 - Yield different results

recently, we ported a heavy-weight C++ project written in VS2008 to VS2013. after a while of playing around, we got it to compile!

Though, when we tested to see the results are "marginally" the same, We noticed that they are far from it.

Some information about the Differences:

1. the differences start only after ~5 seconds of execution
2. Tried Mixing DLLs from VS2008 with VS2013,
   Noticed that only 2/7 cause the issue.
3. We checked that the properties are the same -fp (FloatingPoint convention) and -Zp (Structure Member Alignment)
4. Happens even when leaving Timing aside, and running as independent single-thread.

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I am searching for possible causes for such differences

1. configurations that were default in VS2008 and are in 2013.
2. code schemes that run different under these compilers
3. Suggestions on How can i pin-point the buggy commands that cause it.

Thanks everyone.