Originally posted by Viper007Bond
Sounds like a monitor problem to me (gun is b0rked).
Actually, it may not be as borked as you might think. And it's not the gun. Chances are that the stabilising field for the electron beam has been shifted somehow, or the mesh in behind the glass of the monitor has moved.
First check for external reasons, such as a stron electromagnetic field nearby. Even somethign as simple as a stereo speaker or a florescent light can have an effect.
Either way it can be fixed, assuming that the mesh hasn't moved too much. Inside your computer are two potentiometers, one for controlling the x-axis, and the other for the y-axis. By adjusting these, the electron beam can be "refocused" on the mesh again. Any TV repair person should be able to help you out.