and to add to kwkard post, defragment your hard disk (it's under System Tools). This is when files on your hard disc are split up into chunks and placed in diffrent parts of the disc.
If your hard disc is fragmented then when it looks for the files you want to burn then it has to look for the parts before it can give the information to your burner, the buffer runs out and the burning process is paused untill the computer catches up. If you defragment then all the files are in one peice and your computer does not have to work so hard to find the files.
Ive just had another thought, if your external burner is running on a USB1.1 interface then I don't think the usb channel can give the burner the data fast enough to burn at X24, try burning at a slower speed and see what happens.