Error in burning on ITunes in iMac

Hello experts,
About a week ago, I wasn’t able to completely burn a CD-R on iTunes. I only succeeded to burn two out of 15 attempts. The error message appearing was "Attempt to burn a disc failed. Burn failed due to a medium write error." There has not been such a problem since I bought this Mac, which was a little before three years ago (off warranty).
I have Sony CD-R's and have tried to use several speeds such as 2X and 4X. I didn’t face any problems until last week using similar Sony CD-R's.
I am using iMac 27 OS X 10.5.8 with 2.4 GHz Intel Core 2 Duo. The SuperDrive is OPTIARC DVD RW AD-5630A.
Other information that might be useful:
iMac 24 intel
Mac OS X (10.5.8)
Thanks for help.
