....to MP3?
iTunes' MP3 encoder is fairly lackluster - I would recommend a third-party solution of some sort, preferrably one that uses the LAME encoder.
I use iTunes' AAC encoder, so I won't be too much help on a recommendation, but TUAW did a post about this recently. They don't usually lead me astray, so I'd recommend checking out their post.
TUAW:
http://www.tuaw.com/
Also - I second was Applespider said....if you have access to the CD, re-rip it, otherwise, try to stay away from encoding from one lossy codec to another.