thanks for your help alex
i have updated to the latest version of MC11 and still have 2 problems:
(i) the original problem is now intermittent. about half the files i am converting (ape - mp3) are not playing in traktor, though all of them play in itunes. i am pretty sure the problem isn't with traktor, as i can replicate the problem on different versions of traktor on both pc and mac. also i can convert the problematic ape files to wav, and then compress to mp3 using itunes, and they play fine in traktor.
(ii) all of the ape files have ape 2 tags, but when i convert some of the files, the tags get lost, both in traktor and itunes (all i get is the name of the file under 'track/song name' with all of the other fields being blank). when i reimport into MC though, all of the tags show up fine.
there doesn't seem to be a link between these problems, as they are affecting different files.
i have e-mailed a sample file that won't play in traktor to you as suggested
nick