I was thinking "hardware trouble" until I heard one word: Memorex.
While it may well still be a hardware problem, I'll bet the Memorex discs are a contributing factor. I've had horrible luck with any and everything labled Memorex. They contract out everything they "make" and their quality standards are pathetic. Personally I use TDK, Imation, Ritex/RiData (who also Contract for some of the TDK and Imation discs) and occasionally Fuji. I used to get one coaster aout of every two Memorex CD-Rs. I haven't had a bad burn that can be attributed to bad media yet with the brands I just mentioned.