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.