It could be many factors????
The most common would be that the curcuit board that drives the
LCD Panels is failing , is it still under warrenty.
Sanyo norm give two years i think
Failing that related to the knock has a cable come loose inside the machine ,which connects the i/o board to the drive board.
or then again it could have gone out of convergence but usually this gives a genral ghosting fault of red or green or blue.
Keep me updated