It looks like your CD-Writer is running in PIO mode, even though the PC & Writer are UDMA compatible. In PIO mode, the maximum throughput is about 20x reading or writing and this may be the reason why the CD-Writer has to keep rebuffering beyond 20x. You generally can confirm this by putting in a full data CD-R or CD-ROM (no scratches, etc.), running Nero’s CD speed and doing a read test. If the line goes up to about 20x and stops there across, then the drive is stuck in PIO mode.
Checking if DMA is on is different from OS to OS. In Windows 2000 & XP, you go into the Device Manger by going into the System properties (Rightclick My Computer, select the Hardware tab and click on Device Manger…). Under the IDE ATA/ATAPI controller section, go into the first Primary channel (doubleclick it). Click the Advanced Settings tab and check if any of the devices show ‘PIO Mode’ in the Current Transfer Mode. If not, exit from this screen and repeat for the next channel. If you find a device that is in the ‘PIO Mode’, check that the Transfer Mode selection box above it is set to ‘DMA if available’ and change it if necessary.
Zip drives are the only drives I know of that don’t like DMA or Ultra DMA modes, but I only encountered an old 8x CD-ROM that couldn’t handle DMA. If you find devices set to ‘DMA if available’ (Windows 2000 & XP), and are stuck in PIO mode, then it may be worth updating the IDE drivers or try swapping the devices around. I had my DVD-ROM drive get stuck in PIO mode and resolved this by swapping modes with the CD-Writer.