Ok, so I have read all the previous threads about this issue but feel my situation is a little different.
The screen shakes rather violently (but worse at the corners) and has just started happening.
The difference is that when you change the resolution from the highest (which I have always used) to the lowest the shaking stops and gets relatively worse as I raise the resolution...HELP ME PLEASE
What steps have you taken to try to fix the shaking? Have you checked for any newly placed nearby devices with magnets or electromagnetic coils in them (speakers, power supplies, appliances, compact florescent lights, etc.)? Have you move the eMac to a different location?
In some eMacs, the internal speakers themselves can create video-distorting fields. This would affect one or both lower corners and be most noticeable when playing sounds at medium to high volumes. If the distortion "moves to the music" but stops when you mute the sound, that's the culprit.
The solution is to get a pair of external speakers or use headphones.
I have had this problem for close to two years. It is not the speakers, because I have been using external ones during that time, and there's no strong source of magnetic fields - the external speakers I place at a good distance from my emac (1.25 Ghz, ATI, 1GB RAM, Airport Extreme, original 40 GB HDD, Leopard since about a year ago, Tiger before that). Till late 2007 I was using the machine in NYC, where the problem was already present, and now back home in Ecuador it has continued - but worsened, and just like the original poster reports, only by reducing the resolution does it go away. But before I could stay in 1152x960; then the problem started again after a few months, so I changed to 1024x769. But yesterday, the wobble came back - the difference being that, while the high resolution wobbling involved a flicker like effect, the one at 1024 was more like an oscilloscope displaying a sine wave - there would be wave-like movement of the image, from top to bottom, all across it (so like making explicit the raster scan of the neutron gun), so not localized, as when the speaker is working. But this movement was on a small scale, say about an inch from bottom to top of sine wave.
I have read other posts pointing to the HDD, but even when I boot from an external FireWire HDD, the problem continues. I have tried an external flat panel display, that works fine with a PC up to a high 1440x900; with the eMac, 1) the image doesn't cover the entire display, unlike with the PC, so I have to change horizontal position and display "Clock" settings in the ext. display's own menu (not the eMac's Pref. Pane), resulting in excessively pixelated text, even thought the Pref. Pane says that the image sent to the ext. display is at 1440x900, which is also the case with the PC. Then I start getting the same sine wave rippling of the image, but much slighter, basically lines going up the image, which goes slightly away by moving the notoriously unreliable mini-RGB to RGB Apple converter cable. I know this last points to video card trouble, but you have to be aware that the converter cable is itself a serious source of trouble, and so makes it difficult to truly diagnose the main problem.
The eMac was part of a batch of machines that had some issues with freezes and unwanted re-boots, so I had it sent to an authorized repairman while I was still in NYC, around 2006 or so. I read from the forums that it had to do with the system board assembly, which he replaced.
Some posts in forums here and in other sites regarding the same problem talk about the display dying from old age, but I had another CRT that after about 10 years started losing brightness and contrast, which technicians agree is how they usually show their age. My eMac's display shows none of that. Add to that the fact that my Methuselah Mac 512K Enhanced's rather primitive B&W display kept working fine for more than 10 years, and I don't see how this flicker-wave effect could be a result of aging components.
I hope this long post pays off - a few months more and I will have to dump the machine, as the crappy mini-RGB converter cable will probably die soon after display becomes completely unusable.
The wobbly effect at 1024x768 is not as described on my original post: it is much more complex: the wave is from top to bottom, but it shows as a slight expansion and contraction of the image at the top and bottom, but specially at the top.
