It is a VERY common problem, which we will see more and more as most CD-R dye's were never intended to last this "long"
There is NO solution for CD-R's getting bad than to salve as much data from them before they get really bad...
So you will have to "refresh" your important data every few years to make sure not to lose it
Have a look here:
http://www.cdmediaworld.com/hardware..._quality.shtml