Stability of ordered phases in diblock copolymer melts is studied using a theory of anisotropic fluctuations, which bears a useful similarity with energy band theory in solids. We calculated stability lines of all ordered phases and found the hexagonally perforated lamellar phase to be unstable around the lamellar-cylindrical (LC) phase boundaries. We used fluctuation analysis to obtain dynamical pathways between ordered phases and argue that the observed modulated layered states along LC boundary regions are due to infinitely degenerate fluctuation modes of the lamellar phase.