It could be anything (scratched DVD, bad DVD drive, loose cable, buggy MPEG-2 decoder, buggy Real Player, buggy display drivers, and probably more).
I think it'd be easier if you first tried to play the DVD in something other than Real Player. Real Player doesn't even have internal DVD playback capabilities (it just relies on external decoders) - and I don't like how it hijacks file associations for virtually all media files (and audio CD and movie DVD playback). Do you have another software DVD player? WinDVD, PowerDVD, whatever? Use it instead and report the results.
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