QUESTION Ever since upgrading to Leopard, I occasionally see a thin black horizontal line that runs across part of my screen. The line appears in the Finder, Word, Mail – you name it – so it doesn’t seem to be program based. Also, I didn’t see it when I was using Tiger, so it seems unlikely that there’s a hardware problem (or at least not hardware alone).
ANSWER You’re very probably closer to the mark with the supposition “or at least not hardware alone”. We suggest this because one of Macworld’s Mac Pros running OS X 10.4 (Tiger) has exhibited this same problem on rare occasions. We’ve established that the issue appears to be related to heat and a particular graphics card.
Specifically, if your Mac holds a revision 1 ATI Radeon X1900 XT graphics card, it could display these horizontal lines on your screen. They seem to appear more often when the graphics card is running hot.
It tends to do so more often under OS X 10.5 (Leopard) than under Tiger because Leopard taxes the card to a greater extent, thanks to its many swoopy graphics effects.
To determine the revision of your ATI Radeon X1900 XT graphics card, launch System Profiler (found in Applications/Utilities), select the Graphics/Displays item in the Contents column, and take a look at the Revision ID entry. Those with ID 0x0000 are revision 1 cards. If you can extract your graphics card – as you can from a Mac Pro, for example – switch off your Mac, ground yourself by touching the metal case, and take the card out. Check the card’s cooling fan – in all likelihood it’s caked with dust. Use a can of compressed air to blow away that gunk.
While you have the Mac open, take a look inside and see if there are other clumps of dust that might be blocking its fans. If so, clean them out.
If that doesn’t solve your problem (or if you have an iMac that doesn’t let you access its innards), contact Apple. There are reports of Apple swapping out revision 1 cards for people with troubled Macs under warranty or an AppleCare agreement. The revision 2 cards that replace them seem to work without incident.