CRT monitors are often more prone to outside interference than LCD monitors. Interference can be caused by devices that transmit radio frequencies (RF) (such as a baby monitor or CB radio), or devices that produce magnetic fields (such as a motor in an electric fan), or even by just having a tangle of power cables on or near the monitor.
Radio signals can interfere with the signal going between your computer and monitor, and in some cases can cause horizontal lines to appear, as well as other interference. To reduce interference, move the RF device away from the computer, or move the computer and monitor away from the device.
Moving magnetic fields (such as from an electric motor) can cause flickering when near a CRT screen. A static magnetic field, such as from your computer's speakers, can cause color shifts or incorrect colors in the area near the magnet. This color shift is caused by parts of the screen becoming magnetized. Occasionally the color shift will remain after you remove the source of magnetism. To help reduce or eliminate the color shift, you can use the degaussing option on most monitors. Most newer monitors automatically degauss when you turn them on, while older monitors might require you do to it manually. Most monitors have a button or menu option that enables you to degauss it manually. Degaussing a monitor will de-magnetize the inside of the monitor, removing the color shift. Unfortunately, if the monitor is exposed to the source of magnetism for too long, degaussing will not be able to clear it up, requiring you to repair or replace the monitor.