Sounds like the high voltage power supply is going on that monitor. Time to toss it and get a new one. In any case, repairs are at a minimum of ~100 dollars on CRT's. I've seen dozens of older Gateway Mag Scan monitors do this, all with failing high volt power supplies.

I have found: my refrigerator makes my Sony monitor shake, so does the a/c. Any large electric motor or small transformer near a monitor may cause this. You could try a higher refresh rate, it may help, or substitute monitors to eliminate the computer as a cause. If the monitor seems to do it no matter what, service/upgrade needed..