IMac Intel :: Vertical White And Gray Bars Appear?

Twice in the last couple of days, these white and gray vertical bars/lines would appear on my IMAC screen. In between those times, my screen just went black, but I can tell the IMAC was still on, because it wasn't a pitch black as if the computer was off. Each time, the only way I could get the screen to appear back to normal was to turn off the computer and turn it back on?

This is not good. This is my second MAC. The first one after a year and a half, just poofed up in smoke. (Seriously, that is exactly what happened. I was sitting there, and all of a sudden the screen went black and a puff of smoke appeared from the back.) And now after owning this second one for almost two years, I've suddenly started getting these vertical white and gray bars and the black screen.

Anyone have any idea? I was simply looking up info on a website when these events happened.

Intel Core 2 Duo/ MAC OS X 10.6.3

Information:
IMAC
Mac OS X (10.6.3)