I have a Apple iMac 21.5 A1311. My problem is that i recently changed the hard drive and in doing so broke the glass screen. I have since replaced the screen and put the hard drive in. My probem now is the screen keeps flashing on and off. Is working fine but evey few mins at random goes balck themn bright then black then bright. Its just flashing on and off. Anyone any ideas what this could be.