Image center drifts, bad keystone, jiggles

My Sony multiscan 400PS has been working well for years but now it has developed a problem with image geometry and stability. The monitor is used with a mac mini and with a pc clone with a clone vga card. The monitor exhibits the same behavior regardless of which of these computers it is hooked up to. Furthermore, the behavior is the same no matter what screen size and refresh rate is chosen. No apparent live source of EM interference is around the monitor. The monitor is plugged into a line-conditioning UPS, and it doesn't change if I plug it straight into the wall. Giving it the occasional gentle or not-so-gentle whack on the side doesn't fix it either. For several months now, the image has occasionally started to jiggle slightly up and down, which was pretty annoying, but somehow after about an hour or so it would calm down and go back to "normal". Sometimes the jiggling would be there but in almost imperceptible amounts. About three months ago, I woke up the computer from an overnight slumber and the bottom of the image was bouncing up and down. After repeatedly turning it off and on, I gave up and let it rest for a day or so. Subsequently, the bouncing diminished considerably, but the screen geometry was altered and a bad keystone effect was taking place. In addition, the image had to be "centered" by entering near-edge parameters into the monitor's image options (little buttons that let one shift the image to the right and left). Somehow, after about a week or so, the monitor seemed to get better on its own and went back to more or less normal. Then a couple of days ago, the bouncing effect returned, and once again, it is almost impossible to center the image on the screen. The keystone correction is now set at 100% but it still doesn't fully correct. New monitors are cheap nowadays ($200!) but I prefer not to add to the big pile of discarded stuff prematurely, so if repair is possible and reasonably priced, I'd love to hear about it.