Typically, this is due to hard drive fragmentation (when files are repeatedly created and deleted, it leaves little bits of files all over the hard drive, slowing performance). Every 3-4 months, you should backup all your songs and do a drive initialize (wipe out the hard drive) to eliminate this fragmentation.

To get a specific song to burn, you can do an Optimize and copy. Load the song you're attempting to work with, press Shift-F1 (song menu), and F5 (optimize). Then, go to Shift-F1 (song menu), Page, and F1 (song copy). This will copy the song to a new part of the hard drive and likely will have less fragmentation. Then, load this copy of the song and burn it.