Playing DVDs

With every DVD I play the picture freezes every few minutes while the program continues. When the picture unfreezes a minute or so later the program has advanced to the point it would have reached had the picture not froze. It does this with every DVD and with similar regularity all the way through. When the picture unfreezes, I rewind to the point at which it froze and it always plays well beyond that point until it freezes, apparently randomly, again. It is clearly not a dirty or damaged DVD. What must I do to resolve this?