Buy a CD cleaning disc from anywhere (Office Max, Radio Shack probably, etc.) and spin it in your drive to help clean things that can dirty the laser led lens like fingerprints, peanut butter, you name it...
All CDR and DVDR drives have a finite lifetime. The fact the upper tracks are showing the damage could also indicate the drive may be wearing out.
The Plextor drives we certify and ship have the BEST lifetime of any on the market. I see you have purchased 200 discs from MTU. I wouldn't think that would cause this problem. However, if you have also used other discs in the drive since you bought it in September 2003, say over 800, then you might expect this problem. Each drive is different. There is no absolute warranty for the number of discs that can be burnt.
This also depends on the environment the drive operates in. If your drive is exposed to dust or grit (we have seen some very dusty gritty drives returned to us), then this can reduce the lifetime of the drive much faster than expected. Could that be a problem with your installation and use of the drive?