HA! Well, this may not apply in your case, but further troubleshooting accidently stumbled over the answer to mine - electromagnetic interference. As a former US Navy Avionics tech I have a basic knowledge of CRTs, and I know enough about the magnetic field put out by the deflection yoke not to put a floppy disk on top of a monitor, but it never occurred to me it might go the other way. I have a computer desk with shelves, and the top shelf is over the top of the monitor, and I had recently moved my small 16" flourescent light fixture onto the shelf just above the monitor. Couldn't figure out why sometimes I got the jittering and sometimes didn't, so I went to look at the cable connections, and when I moved the light to look under the desk the flickering/jittering immediately stopped! The ballast inside the light fixture was about 2 inches above the upper right corner of the monitor, and apparently the ballast from even a small flourescent light puts out a strong enough magnetic field to cause that jittering. Might be worth checking out, if you have anything near the monitor that might produce a magnetic field.