I have been curious what causes this too. I had a CRT pc monitor that developed it early in life but I never bothered to do anything with it as it was only a "secondary" type of system that was hardly used.
If this came out of a Megatouch game, I've seen this symptom happen if the frame ground of the power cord was NOT connected or if the ground connection on the monitor main board was not firmly connected by either the mounting screws or the tube ground wire.
Yep, Megatouch! Removed all grounds, cleaned everything up for contact and still have the line. I must be missing something, but I can't figure it out. The picture is sweet and everything respondes well. Not sure whats next.