In general the lower the speed, the better the copy. I would guess that some error correction scheme in in place for the copies. When speeds go higher, the error correction get's call up more, so the data is less uniformly written. This is just a guess.
I notice that my slow work computer (933mhz P3 w/5400rpm ide) must copy at 2x in order to make a good copy. My fast home computer (athlon 1.8 w/twin 15K scsi drives) however reliably makes good copies at 24x and 40x.
1. On your main board you will have two ide connectors. Usually you will have one Y cable coming out of one of them. The destination ends of the Y go to cd rom and cd writer. You should go and buy another IDE Connector cable, connect this to the second mainboard ide controller, and to either the cd or the cdr. The effect being that they are on separate ide channels and won't have to share a single cable. www.outpost.com sells them for 2-3 bucks. This is important.
2. Disconnect any and all unnecessary virus alert, email, or fax monitor software. These things will periodically use up a little cpu for their timer based services. The virus stuff expecially.
3. Don't use your computer while it is copying.
4. Buy high quality cdr's. I use TDK's. I have had consistently poor performance from 'house brand' cdr's. The cheap cdr's work ok for low speed (8-12x copying).
5. (extreme) Get a very fast hard disk for your system, some extra memory, and a fast processor. This will cause it to have fewer 'pauses' while the processor is servicing other programs.