If it's a bright vertical line in the center of the screen with nothing else on the screen, that sounds like a problem with the horizontal sweep for the video. Sometimes you can "fix" this if you just whack the side of the case really hard while the Mac is on. If that fixes it, even temporarily, it means there's a bad solder joint on your analog board somewhere. Check L2, C1, and especially J1 on the analog board.

See Larry Pina's Macintosh Repair and Upgrade Secrets, page 83, for more details on a vertical line display.