I think it could be that either you are using poor quality CD's or that you need to update the firmware on the DVD drive itself. Try some high quality CD's and if that doesn't work then look at: click here for an upto date firmware for your drive.
Please bear in mind though, bad flashing could destroy your drive. Carefully read the installation notes of your firmware before you upgrade. The Firmware Page and the drive manufacturer can't be held responsible if it goes wrong. This is at your own risk!!!!