CLoneCD Erratic disc read problem

I was making a backup copy of a set of three music cds for the car. Two of the discs copied fine, but the third has some strange distortion on tracks three quarters to the end. I noticed when reading to image that when the disc gets about 70% done, the read speed drops and restarts erratically from then on to the end of the disc. It would appear that this is about the point at which tracks become distorted when you playback.
I have tried running in different drives and different machines, but with the same results.
Can anyone tell me why one disc out of three would do this?
Many thanks.