This kind of problem is usually the result of interference from another device affecting the image. If the image appears totally scrambled and unviewable, this is usually a different problem . Examine the area around the monitor and remove any devices in the proximate area that could be interference sources.
Common culprits are fans, motors, transformers and especially other monitors.
Putting two monitors next to each other usually will cause each to interfere with the other. Try plugging the monitor into another power circuit to see if this causes the problem to go away. If it does, you may have line noise problems . Try changing the physical location of the PC within the room. This will sometimes solve the problem by moving the CRT out of the way of some unknown interference source.