I would not call mine SOLID AS A ROCK because I also have a 1980sxi, the one with the square-metal foot-pad. The 1980 is SOLID AS A ROCK. That monitor doesn't budge.
The larger models with the triangular feet do behave like bobbing heads but such a description is an exaggeration. If you nudge the screen strongly, it has a bobble effect.
But who nudges their screens (except to turn off the LED light)? My workstation is secure and it doesn't move, therefore, my monitor doesn't move either (unless I press the "off" button).
I wouldn't return it unless you feel you have something extremely abnormal (which I doubt).