Most likely cause for the buffer not staying full is your computer can't read the data fast enough to keep the buffer full. For example, if the computer needs to page to the swap file, or the drive is too fragmented, the buffer fluctuates because of the extra disk reads required to get the data.
If you are copying on-the-fly directly from one DVD to another, don't. The second disc reader might not be able to read fast enough to keep up with the burner. Copy the DVD to the hard-drive first and then burn it from the hard-drive.
1 - Make sure your hard drive is set up for DMA. PIO will slow it down too much.
2 - Don't multi-task during burns. The burning software needs all the memory it can get.
3 - Defrag your harddrive. If you are burning from the hard drive, the file(s) may be too fragmented to read quickly.
4 - Upgrade your memory if you have less that 256MB.
5 - Make sure your computer is free of viruses and spyware. They hog resources and slow everything down.