Unit Tests Fail when "Run All" in Visual Studio but passes individually

I am having this wierd problem with MOQ Unit tests.

I have around 636 unit tests written using MOQ Framework in VS 2012. When I run them project-wise and/or individually, they pass successfully. but whenever I select Run-All they fail randomly showing different errors.

Most of the methods show IO.FileNotFoundException.

I looked Here here and here but no clue yet.

Please help me out here.