You can always try taking your disc down to the local DVD rental store and ask them to run it through their resurfacing machine, most have them and they have saved my arse a few times, unlike those home resurface machines these ones polish the disc to look like brand new and often will allow a previously problematic DVD to read perfectly - even ones that didn't 'appear' to be scratched.

So can you browse the problem DVD in question at all? i.e. can you see the filenames or do you get a read error straight away?