Also, the actual drives are subjuct to age and contamination. Try one those cleaning disks (if you can find one) that you put few drops of isopropyl in and then insert it. Don't overdo it!
The spin rate of the disk can also be out of spec. This is harder to diagnose. I have seen photocopy patterns for use with a strobe.
You could also swap the drives and then look for a replacement.
