I've experienced some odd behavior with my CD-RW disks. Using Memorex 24x media.
What happens is that by the 4th or 5th time I re-write the disk it will slow down to 10x (on my NEC ND-3500AG) or 12x (on my Lite-On LTR-52327S). The first 2 or 3 times I re-write the disk it works perfectly at 24x. I'm using Nero 22.214.171.124.
Anybody have any ideas???