You can only get as good as you have got!! (what terrible english that was!!) - you could scan the original using CDSpeed to show you the quality of the disc, but I suspect that it may be crappy media. Make sure the disc is very clean, dust, finger prints for example - you could try something like Isobuster - that will recover things that other programs won't - but you have to pay for it.
Or try it in another DVD reader (at a friends for example) different drives have better error correction. (my Sony/Liteon drive seems to be able to read a lot of crap that my toshiba just rejects)
Make sure for the copy you are making that you are using decent media as well.