Weird. Sometimes get horizontal lines at boot, sometimes no.

Here's an odd one. Odd, because the problem isn't permanent.

I'm running a Macbook Pro Core 2 Duo 15" from 2006. I've installed 3 different OS - Snow Leopard, Lion and Mountain Lion - and when they run okay they run well.

But, occasionally, at boot, when I get to the Apple logo and spinning gearwheel, right at the end of the boot the display shows a lot of horizontal lines, and boot does not finish. I reboot again, two or three times, and get the same problem.

But then, on maybe the fourth reboot, it all boots fine! And no more horizontal lines! And the computer works fine. Weird.

When it boots and works, after a short time the fans come on and run pretty fast at regular intervals, even when the computer is idle. It gets pretty hot near the Magsafe port, but then everything cools down after a few minutes.

Any ideas greatly appreciated.

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Do the lines start at the bottom and roll upward?

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No, not like that. The lines appear all of a sudden near the end of the boot and are from side to side. They don't roll. This happens with 2 boots, then at the 3rd boot the computer boots fine. I get a message that I did a forced shutdown, but the desktop appears and the computer works fine.

Also wanted to add that, as suggested elsewhere, I deleted Library caches and rebooted. It booted fine, and has since booted fine the next two times and worked properly.

I hope it was only a software problem, but who knows at this point.

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Sounds like a GPU issue, which is common on this model. You can try reflowing the board to get a bit of extra life out of it (temporary fix), but I would recommend just selling it for scrap on eBay.

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Reflowing what on the board? Do this if it is the nVidia 8600 GPU?

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Yes, the GPU. Some people have success by heating the chip (putting in an oven, for example), but not at solder melting temperature. It will happen again though, there is no permanent fix for this, since the chips are not made new anymore

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