dvd backups skipping

[splashpot]

Disclaimer: I'm not going to deny that I'm doing anything illegal for the simple reason that it's not going to convince anything who believes otherwise. Since my question has nothing to do with illegal activities, I hope this is ok. If not, delete away.

I started backing up DVDs but I noticed that a large majority of my copies skip a lot. I've come up with several hypothesis's for why this is happening, none of which I'm sure has any plausibility.

-Old DVD burner. I'm using a 2 year old Dell Inspiron 6000 and the DVD burner that came with it. Not sure exactly why this would be a problem, but maybe old technology is more error prone?
-Old DVD+Rs. I found a stack of DVD+Rs sitting on my bookshelf in the basement. No idea how long it's been there. How fast do these deteriorate? If this is the problem, it should be solved by buying new DVD+Rs, right?
-Original DVDs are too damaged. Some of the DVDs I'm backing up have scratches on them. I figured if my computer could read it well enough to rip and burn them, they'd be fine but maybe not?

Are any of these possible causes? Does anyone know which is most likely and is there a way I can test to find the cause? Is there anything I haven't considered?