I just got a Mac Plus off ebay which is in excellent condition with original boxes and manuals, but the screen will be on and then go blank. I tapped the top of the box and it came back on. But after awhile it goes off again. I opened it up and check the connections that I could find, but it didn't help. If I move the Power/Sweep board when the screen goes out, it will come back on. But after awhile, even moving the Power/Sweep board doesn't help. Any ideas?