I had to try six different drives before I got one that would read a particularly annoying DVD.
Of course, I have a pile of a dozen or so laying around. No guarantee it will work in your case, but if you can find more drives...
I have an Acer CD lens cleaner with the little brushes on the disk that does a pretty decent job of cleaning a drive, maybe try one of those. I think Wal-Mart sells something similar, or Staples, Office Max, Best Buy, the net... They are good to have regardless of whether it fixes it or not.
The drive needs to be in near perfect condition to read a dodgy disk, if it can be read at all.
I guess there is a difference in Linux between ddrescue and dd_rescue, and dd_rescue is supposed to be better. I haven't used them but many refer to dd_rescue as being their salvation.
They also sell CD/DVD repair / polishing machines halfway cheap on the net, but no certainty there either. If the data is important enough, it might be worth trying.
http://www.google.com/products/catalog? ... IBEPMCMAU#http://www.google.com/search?q=cd+dvd+d ... CDkQrQQwAgThere are other brands not listed there. You might want to Google some reviews before making a choice.