did u install the motherboard correctly? there is a chance that the motherboard is not installed properly. what i mean is when u put ur mobo to the casing, did u put any "stand" (sorry for my bad english) to hold the motherboard? check if the stand is installed properly. once before, i having the same issue and after several days, finally i realise that two of the "stands" causing some static electricity that my mobo restart instead of shutdown! p/s: cant really describe the whole situation because of my bad english