I have an Intel iMac 2GHz Core Duo with 2GB of 667MHz DDR2 SDRAM on OS 10.5.6. The video card is a ATI Radeon X1600 with 128MB of video memory (built in).
I've been having this issue for some time but have mostly been ignoring it since it is more of an annoyance than anything else. Randomly, while using the machine, some strange looking horizontal lines will appear across the screen of the Mac. Sometimes this is limited to a single window (Safari or some other app) but it also appears across the desktop from time to time.
If I resize the window it is in or change the desktop size down from the max and back again, the lines disappear. It even appears over the screensaver. Screenshots do not catch the glitch - the image appears normal.
The GPU diode reads anywhere between 119 and 130F. This isn't a machine I use for a lot of taxing stuff... generally just checking email and remoting in to work.