Sorry for my late reply, but I have found the solution for my problem in Luna. It looks like upgrading libmutter0 from 3.4.1-0ubuntu99~elementary7 to 3.4.1-0ubuntu99~elementary8 and reinstalling gala is the solution for the problem in Luna. Gala is running smoothly ever since.
Sorry for my late reply, but I have found the solution for my problem in Luna. It looks like upgrading libmutter0 from 3.4.1-0ubuntu99 ~elementary7 to 3.4.1-0ubuntu99 ~elementary8 and reinstalling gala is the solution for the problem in Luna. Gala is running smoothly ever since.
thank you everyone.