This usually happens if the iso download is corrupt (which you said isn't) or the DVD-media you are using is bad. I recommend try using a different brand of dvd (I've always had success with sony and samsung brands - memorex not so much) . Also, it could be something as simple as the lens on your drive may be dirty, using a lens cloth to clean might help if that is the issue. If all else fails, go with a USB boot (My preferred method
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