My iMac is old. It's an iMac core2 duo 1.83 that was introduced in 2006. I've had it since it was new. About a year ago I had the display only showing hundreds of colorful vertical lines and so I bought a display adapter and hooked it up to an old external display I had laying around. Everything worked great until a week ago. This time when hitting the power button on my iMac I heard the startup chime but noticed that the external display was staying black. I still see the same vertical lines on the iMac display and can hear it's running but my external display stays black. I tried hooking up my external display to a completely different computer and the display works fine. It's as if there's no connection between my iMac and the external display anymore. Any ideas?