Sounds very much like the files themselves. Try re-ripping them if possible, or check for odd samplerates (non-44.1 kHz) or very large tags (embedded album art or long comment field)
Otherwise, you may be lucky and someone who knows what they are talking about will offer to take a look at the actual files in question.
Then it normally goes one of two ways:
The files have been ripped/encoded in some standards non-compliant way = no action
The files conform to standards but expose a bug in Rockbox's codec/playback implementation = bugfix!