Liteon 411SX External - Newbie Needs Help

Hi Guys. Great forum.

I have a Liteon 411SX External Drive. I’m having a problem with some of my backups. The picture is breaking-up towards the end of the disc.
I’m using Ritek minus discs (purple/silver front) & dvdshrink and copytodvd software.
Is there anything I can do to bet a better performance?
What media is the best (cheap-ish)?