This problem is a fairly common issue with the a1278. I have been dealing with it on and off for the past year. It is related to the LVDS cable being either loose or gradually failing. It is a somewhat challenging fix but the part costs less than $50. Applying pressure to either the logic board or the screen, depending upon where the cable is loose, merely re-seats the contacts temporarily. It is really a poorly engineered component and Apple should be ashamed this was allowed to pass. I am waiting for ifixit to stock the cable as I do not trust sourcing the cable from some of the discount dealers that are out there.