One really bad cause is a missing ground on some socket or device. You can research and find articles where folk sent the tv, pc or other thing up in smoke, flames and simulated the starship Enterprise command deck under fire from the Klingons over this area.
VGA is analog so any fault can show up like that.
I have a VGA cord to my tv from my computer and the cable cord for the cable plugged into my Sony but when they are in together there is interference (lines running from bottom to top) on the PC INPUT!! It is driving me absolutely crazy because I just can't figure out what the problem is. The people who I got the VGA cord from even gave me a different one because I thought maybe it was just the VGA cord but the new one I got is still having the same effects. As soon as I unplug my cable while on the PC INPUT the lines are gone but I can't go back to cable!