Sounds like a pretty classic sign of a display failure. Given the age of the system, and the fact that repairing this means replacing the display... Either learn to live with it until it starts throwing a party with a bunch of its friends, or buy a new system.
Recently, a vertical black line (maybe slightly dotted) appeared running down the screen of my iMac G5 (Power PC processor). On the left of the line the screen appears fuzzy while on the right it's OK.
Has anyone experienced this before? Hope to hear from someone who has.