Looks like a broken flex cable or soldering... it might be not something easily detectable with a naked eye. Take a good magnifying glass and carefully examine the flex cables for any cracks, as well as the state of soldering joints, especially in places where the flex cables connect to the mother/daughter boards as well as on the D-Pad board (which receives most mechanical pressure).
If you dare to, turn on the phone while it is taken apart and try to VERY CAREFULLY "swing" the cables with a non-conductive tool to see if it has any effect. Aslo try to apply a bit of pressure (again, VERY CAREFULLY to avoid any damage) to different areas of the boards itself to locate the most probable area where the defective joint might reside.
But, again, unless you're skilled in electronics hardware repairs, the best you can do at home is identifying a potential problem.