I am having no problems burning MP3s directly.

How fast is the burner? If it is really fast, make sure the media you are using is rated to be burned at those speeds, and see what speeds Nero is giving you. One cool thing Nero does, though, is that it detects this automatically when the burn dialog comes up. Have you been moving the speed up to max each time if this is the case?

Other than that, I can't think of anything else... sorry