|If things change when you tap the case it is almost certainly a connection problem of some sort. When it is running, gently moving or even tapping cables and connector ends with an insulated object (such as a toothbrush handle) can help show where the problem lies.|
One thing you could try is powering off and using a soft pencil eraser on all edge connectors - sometimes the slightest movement can shift these onto oxide. RAM can be the culprit and after cleaning the edge connectors pull the sticks in and out a few times to clean the sockets. Same can go for video card if separate, and SATA power and signal connectors too.
Always pop back and let us know the outcome - thanks