OSX screen corruption black lines

My brother's iMac has thin black lines across the screen. It looks
like a software issue, as the lines are per-window and scroll with the
window's contents. Sometimes Firefox gets chunks of its display
jumbled up. The logon screen has large blocks of random pixels on it,
and desktop icons have strips of random pixels as well. Any
suggestions? Sorry I don't know what version of OSX it is, but it was
bought a little over a year ago if that helps.

Phil.

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