M991 (Dimension 8100) Monitor problem

About every few days or so my monitor screen (m991) shifts to the left by about a 1/2". At the same time the display is very squiggly. All I do is tap lightly on the top the monitor and the problem corrects itself. The PC and monitor are about 2 1/2 years old.

It appears to me that either it has a faulty component or a loose connection somewhere. Has anyone experienced  a similar problem? If so, what was the cause?

I would  much appreciate someone to respond.