It could be the tags or the encoder.
Strip the tags from the files using foobar2000, re-tag them, and try adding them again to iTunes. And when I say strip the tags, I mean actually remove them, don't just delete the text using iTunes because that's not the same thing.
If that doesn't work, it's probably the encoder. And if that's the case, there's really not much you can do besides hope to find another encode.
...well, you could transcode them but that's a bad idea as it'd just diminish the sound quality of the files. 