Here are a few things to try.
1) Try burning the data CD's at the slowest speed possible.
2) Try using the 'other' SCSI connection on the back of the CDR (power all units down first, change connections & reboot).
3) Try different media. CD media is the biggest perpetrator of problems like this. Try a higher quality CD, like Mitsui, HHB, Apogee, etc.

I would suggest that after you get everything backed up from the internal hard drive, that you perform an "Drive Initialize" and choose the "Physical Format" option. This will completely erase your hard drive, which can drastically increase drive performance. This should be performed every 2-3 months.