Checking the supply voltage is certainly a good recommendation. You may discover that the voltages are fine, however. If that's what you find, note that a not-uncommon source of the symptoms you describe is magnetic in origin. The magnetic fields emanating from the drive can easily perturb the electron beam, causing the raster to execute a characteristic hula dance. If that turns out to be the problem, your options are limited. Either reorient the drive (difficult), replace the drive with one whose fields are weaker or oriented differently (a crapshoot), or live with it.