That is weird, no clue why that would happen. Try burning a new Dvd, it's possible maybe you burnt out the dvd? haha By that I mean maybe the heat caused it to warp. Happen to one of my Dvd's. I had an issue with Kalyway's Dvd. When I put it in my dvd drive and tried to boot with it, it wouldn't boot had some weird error, tried it like four times. I forgot I left the dvd in the drive and turned off my computer, when I got home from school the next morning, my friend was going to use my computer for hw but somehow it booted from the Dvd. And now it works everytime, weird to me.

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CPU: Phenom x3 8650 @2.54 ghz
Mobo: ASUS M3N HT-Deluxe 780i SLI
3 GB 800 OCZ RAM
Lawless 10.5.4 w/ Voodoo Kernel